I'd leave it in for 2 hours for corys (actually I drop mine in and then go to bed).
If there are only a few fish break the wafer into half and drop half in each end of the tank as Colin suggests.
Place them in at night just before you turn out the lights. The cories will find them and the betta will probably go to sleep. In my experience, they’ll be nothing left to have to clean out in the morning.