In normal times, I would say that my comfort zone is typically anything which involves one more feeder flight after the one I have booked which would enable me to arrive at the airport for my next flight in case mine gets cancelled. (So if my second flight leaves at 5pm, I'll book an 11am arriving at 1pm IF there is also a 1pm arriving 3pm which I can potentially book should my 11am get cancelled).

I would say that in the current covid climate, this is all the more important because flights can and do get cancelled all the time, and if your flight is cancelled, with two separate bookings, nobody will make efforts to help you.