I intially did them as soon as they were available, at whatever time of day they showed up, but b’se of the SRS timing, that meant some of them were coming up for review in the middle of the night.
Then I read jprspereira’s advice (now incorporated into this guide) and I set a regular schedule for lessons and reviews for myself, so I now only do a set number first thing each morning along with reviews, with the other 2 sessions only for reviews. The bulk of your reviews will end up in the 2nd review-only session.
P.S. See #4 of the guide regarding SRS timing and setting your study schedule