Regarding the "last hour" language, dispensationalist Thomas Constable writes that it is the entire time between the ascension and the return of Christ:
In the drama of human history, all of John's readers, including ourselves, play our part in the last act. Throughout the New Testament, the writers regarded the present inter-advent age, after the Incarnation and before the Lord's return, as the "last hour" or the "last days." This is the final period before the Lord Himself breaks into history again.
Following this period will be the two-stage "New Era" or end-times proper: the Tribulation and Millennium.
Regarding "antichrists," Constable sees John drawing a distinction between the Antichrist and antichrists:
Even as John wrote, "many" little "antichrists," people who exalt themselves against God, had arisen. John saw this as evidence that the appearance of the Antichrist was not far away. "Antichrists" are those who oppose Jesus Christ and His teachings, and or people who profess to be the Messiah.
The Moody Bible Commentary describes these "antichrists" as "professing Christians who rejected orthodox Christianity's teaching on the identity of Jesus Christ" and people who "abandoned the church." Ultimately, they "were forerunners of the antichrist," the man who Moody identifies with the "man of lawlessness" in Mark 13:14 and 2 Thessalonians 2.