i've thought about this a lot and i think the two main aspects are travel time and how long it would take to get to the destination otherwise (mostly considering walking and maybe cycling, because drivers are probably not choosing public transport because it's faster unless we're talking about high speed trains).

Like if you're taking the bus to a town 30km away then you're certainly not walking and biking there would take hours, so hourly buses is acceptable and half-hourly makes you go "oh man this is great".
But if you're taking the bus a couple kilometers just to another part of the city then you can totally just walk and cycling might well be slightly faster, so even half-hourly buses becomes.. borderline acceptable for commuting to and from work, maybe. You really want AT LEAST every 20 or really 15 minutes for shorter distances, otherwise what the fuck is even the point?