In our nets everyone gets exactly the same amount of time batting (and bowling to some extent) but we have tried a couple different ways of who goes first. Originally it was always batsmen bat first and bowlers bowl first, as the batsmen finish their turn then all-rounders and then bowlers get to bat, bowling was a decent mix of everyone else who was not batting, we even had 3 lanes of fast bowlers, spinners and medium for each lane so if a batsman had 9 mins to bat he would switch lanes from fast bowlers to a different variety after 3 mins so all 3 batsmen batting at the same time get a good mix.
That was all good but then the problem was guys who turned up late and just padded up or loitered around, so we changed to first come first serve. Guys show up, their name goes on the list and we all bat in that order, we let off the different bowlers in each lane and batsmen switch as that was wasting time, we just do a good mix of each bowling type in each lane and you bat your entire time in one lane. Its pretty tiring when you have 10 bowlers lined up one after another to come at you non-stop for 9 mins.