I only know with high street stuff, and that usually says on the packet how long it's expected to take. You HAVE to follow it exactly otherwise it won't clear up as quickly, and you might give it a chance to flare up again.
To perhaps help the process and prevent reinfection you need to prevent your feet developing the hot and wet environment that the fungus thrives in with regular foot washing, cotton sock changes, and religeous between the toes drying. Personally, I found that the cream didn't help matters when I'm drying to keep sweaty feet dry, it just made them sweaty again. It's really expensive, but the stuff you just use once seems far superior.
Another problem you might be having is with communal showers, where someone else with foot fungus keeps using the shower and infecting and reinfecting everyone else, not so easy to deal with.