For me it depends on several factors. If I know the flight back is likely to be the same a/c, and know there is a good chance I can take a later flight if misconnect (either through flexible ticket or playing the status card or airline allows same day standby) then I don't mind having short time between last arrival and first departure on separate tickets. Some airlines even allow to check in all flights for the day at once (or OLCI even without finishing prior itinerary), even if on separate tickets - these are even better since don't have to worry about the last flight on first ticket arriving past check in cut off for first flight on second ticket.

There are other situations where it is riskier and I'd allow longer. For example instead of taking the first possible flight on ticket 2, I'd book the second or third possible flight. For multi-day running I also like to build in a buffer every few days - design the bookings so if it all turns nasty I can skip stuff (or have a safe reroute for little or no cost) and get back on track again.