The standard rule, going back to B/X I think, is to roll 1d6 to check for random ( on a 1) encounters every other turn. Instead, I’ve been rolling 1d12 (still just on a 1) every turn because that should create the same overall frequency while not arbitrarily ensuring that the time between random encounters is always a multiple of 20 minutes.
But I also realize that sometimes the old rules do things a certain way for reasons that aren’t obvious. Can anyone think of an advantage to only rolling for an encounters on even numbered turns? Maybe an administrative reason?
It’ll only really feel different if you pull two wanderers in a row at 10mins. I’d consider it more deadly in that rare case, but the only gameplay effects are ones you’ve likely thought up already.
From a delver’s perspective, 20 minute intervals promise some breathing room to explore, rest after combat (always a turn), do something after a parley that results in cooperation, or retreat further after fleeing.
You could try it both ways, but you won’t start to see the differences until you roll a few hundred checks.