When you get tired, or want to just stretch out a bit, stop and do so. I think this is a very individual sort of thing based upon factors that are only applicable to the individual cyclist. I find at the start of a ride I can go for a much longer time that at the end. So if i was doing your 80KM maybe I don't need to break for the first 40 km, and then more frequent for the remainder of the ride. Food I do the same way, but thats just me, maybe you want to eat as you go along just to avoid the bonk.