From my experiences, it takes at least 3-4 within an hour to even feel that light, bubbly feeling acid gives you.
5-6 and you will actually get visuals, its not so much actually seeing things as much as vision blur and light tracers
I've never actually taken more then i would say 5 in less than an hour, but the last time I dropped I probably did 10-12 within a 3 hour punk show
Then again, Im more used to the feelings, and my tolerance to most substances are higher than most