The cyclic group generated by e.g. RU has order 105 (so 210 total turns or 105 of each side, alternated). If you have some math know-how, check out [1]. If you don't, take my word for it: when I was a teenager playing around with cubes, I once had a similar experience trying to do the same thing you did - when I went relatively quickly it never returned to the solved state, but when I was very deliberate about each turn, I got the 105 result (not by counting back then, but by rough time estimate given the figure I just looked up). Both you and I probably accidentally threw in one or more double-turns (like a U2) in there, or undercounted and gave up well before the cycle had completed (I, too, had thought I'd made "hundreds" of moves).

[1] https://faculty.etsu.edu/gardnerr/4127/algebra-club/rubik-ta... - slide 41

It should take 105 repetitions:

https://alpha.twizzle.net/explore/?alg=%28U+R%29105

Unless by "same direction" you mean "opposite direction", in which case 63 works:

https://alpha.twizzle.net/explore/?alg=%28U+R%27%2963

It takes something like 60 turns or so in my experience. you probably messed up while doing it.