In the four Gospels of the usual canon — Matthew, Mark, Luke, John, — only two of them mention any measure of time:
Then He came to the disciples and found them sleeping. He asked Peter, “So, couldn't you (all) stay awake with Me one hour? […]”
― Matt. 26-40, HCSB
Then He came and found them sleeping. “Simon, are you sleeping?” He asked Peter. “Couldn't you stay awake one hour? […]”
― Mark 14-37, HCSB
In the narratives of those two books, he goes away to pray twice more. In the book of Luke, only the final wakening of the disciples is described, and in the book of John their stay in the garden is mentioned but very briefly.
Estimating from the time of year — near the vernal equinox and on the summer side of it, — it could've been anywhere from a modern hour to half the night.
Do we have some more precise records which would indicate a timeframe there?
E.g. when he was arrested, or perhaps some non-canonized documents describing when they would've gone to the Mount of Olives?