the time is 24 hour.
as I say, the footfall is per-mayor per-settlement. not per-beacon per-settlement.
if player A has two beacons X and Y in the same settlement S, and player B has permissions on X but not on Y, then whether player B generates footfall for player A depends on which beacon he enters first basicly.
assume player B hasnt played in over a day so definitely no cooldowns are applicable, and player B enters beacon X, it will not provide footfall for player A (permissions fail) but 24hr cooldown will still begin for the (player B, player A, settlement S) tuple, so even if player B walks over to beacon Y now, he will not generate footfall for player A even if he has no permissions, because the cooldown is still in effect.
if player B had first walked into beacon Y instead, then he would have generated footfall for player A
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the bug I would say, is actually that player B can generate footfall “at all” (by walking into beacon Y first) since player B does have permissions on a beacon owned by player A in the settlement, just not the one he happens to have walked into first.