~6 months is a possible answer, as reviewed by SLM, and supported by the custom of Jewish priests beginning their ministry at the age of 30 (see Numbers 4:3). Because Luke uses "about" 30 years of age, and is unclear whether this timestamp refers to Jesus' baptism, the beginning of His ministry, or somewhere in between, I'll offer a range of possibilities. -- **Beginning of John's ministry** The single most precise chronological statement in the entire New Testament comes when Luke introduces the beginning of the ministry of John, in Luke 3:1 > Now in the fifteenth year of the reign of Tiberius Caesar It has been oft-debated what is meant by this phrase...but it needn't be. There are a variety of reasons people have sought to make the 15th year of Tiberius something other than the 15th year of Tiberius (such as a priori commitment to a particular year of Jesus' birth or death), but the evidence is rather straightforward: - Tiberius was confirmed emperor by the Senate in September AD 14 - Every ancient Roman historian reckons Tiberius' reign non-inclusively from after this time - Every datable Roman coin reckons Tiberius' reign non-inclusively from after this time It is unclear whether Tiberius' regnal years were reckoned from *September or from the following January (Roman New Year's). Therefore, John's ministry began no earlier than September AD 28, and no later than December AD 29. (see my video [here][1] for a more thorough presentation of this argument) -- **Beginning of Jesus' ministry** I argued [here][2] that there is compelling reason to believe Jesus died in April, AD 33. Arguments from the Jewish lunar calendar, from the number of Passovers in the Gospels, from the political situation at the time of Jesus' trial, from the Babylonian Talmud, and from lunar eclipse calculations are used. The Gospel of John records at least 3 Passovers in Jesus' ministry (2:13, 6:4, 13:1), and possibly a 4th (5:1). If the Spring of AD 33 was the 4th Passover of Jesus' ministry, the 1st Passover was in the Spring of AD 30. If AD 33 was the 3rd Passover, the 1st Passover was in the Spring of AD 31. John 2:11-12 indicates that Jesus began performing miracles publicly "not many days" before the Passover was at hand. Jesus' ministry began close to Passover (there are arguments that this was up to 6 months after His baptism, but the chronology here is approximate at best). -- **Range of possibilities** The shortest possible interval between the beginning of John's ministry and the beginning of Jesus' would be to take the latest possible date for the beginning of John's and the earliest possible date for the beginning of Jesus': December AD 29 to early Spring AD 30 = **~3 months** The longest possible interval would be to take the earliest possible date for the beginning of John's and the latest possible date for the beginning of Jesus': September AD 28 to early Spring AD 31 = **~30 months** -- **Conclusion** John the Baptist had been preaching for between 3 and 30 months when Jesus' public ministry began. Given the degree of fame John held, I suggest the very lower end of that range is quite unlikely. *** *** **Appendix - Coregents & Coins** *Coregency of Augustus & Tiberius* I have not taken a position on whether there was or was not a co-regency between Augusts & Tiberius, because I do not believe the available evidence allows us to conclusively rule out one or the other possibility. Suetonius appears to support the view that there was a co-regency, Tacitus & Dio Cassius appear to oppose it. But none of them pay much attention to it either way. The proposal I have outlined above works whichever side of the co-regency debate we land on. *Coins* There is value in distinguishing between double-header coins & double-dated coins. A double-header coin includes the figures of two individuals; a double-dated coin provides the year the coin was minted on two calendars. A double-header coin *may* provide supporting evidence for a co-regency...or it may not. Other powerful Roman figures were memorialized on coins without being Emperor (e.g. Tiberius' mother, see [here][3]). The trouble is, without a date that can be paired with a known calendar, a double-header coin does not allow us to answer the central question: from when was the reign of Tiberius reckoned? A double-dated coin is historical gold (pun intended). It provides the year on two different calendars. If one calendar is known, it allows us to understand/utilize the other. It's kind-of like a Rosetta Stone for coins. There are a few known double-dated coins from the reign of Tiberius (e.g. [here][4] and [here][5]). Both of these coins provide a regnal year for Tiberius and the *Actian year*; a count from the Battle of Actium. The Battle of Actium is a significant datum in Roman chronology, because we know with great certainty that it occurred in September 31 BC. Pairing the Actian years with the regnal years of Tiberius on these two coins shows that Tiberius' reign was reckoned from *after* the death of Augustus, regardless of whether or not there was a co-regency. As such, I maintain that the ministry of John the Baptist, which commenced in the 15th year of Tiberius, began no earlier than late in AD 28. *Miscellaneous* My channel has a video reviewing the relevant coin arguments [here][1]. For an in-depth discussion of competing numismatic claims, see [here][6]. The argument for dating Tiberius' reign from a co-regency is indeed popular--but it is relevant to consider why it is popular. Far and away the most common motive for dating Tiberius' reign from AD 12 (or 11, or 13) is to develop a chronology that aligns with the [Schurer hypothesis][7] for the timing of the death of Herod the Great. Though Schurer's views were politically popular in the 1890s, he had to make several significant assumptions to get the math to work, and his calculations have been repeatedly challenged in more recent years. My own video series on the chronology of Herod discusses this [here][8]. *** *Tiberius' reign was probably reckoned from September AD 14 or January AD 15, though I acknowledge there's an outside chance it was reckoned from August AD 28 [1]: https://youtu.be/XsJqhJphSkQ [2]: https://youtube.com/playlist?list=PLGACqQS4ut5jZnDaAdEpShgbkiyttyMa1 [3]: https://www.forumancientcoins.com/catalog/roman-and-greek-coins.asp?vpar=384 [4]: https://rpc.ashmus.ox.ac.uk/coins/1/4270 [5]: https://www.vcoins.com/en/stores/harford_coin_company/183/product/roman_provincial__syria_seleucia_tiberius_ae_28_or_as_16_ad_rpc_i_4330/495840/Default.aspx [6]: https://www.academia.edu/44668230/The_AD_26_View_of_the_Fifteenth_Year_of_Tiberius_and_the_Coins_of_Thesaurus_Morellianus [7]: https://youtu.be/0MzXhJqVowg [8]: https://youtube.com/playlist?list=PLGACqQS4ut5j6BkvLX7Ic07qKBJUh6CH-