Depending on the yeast you used and the amount pitched, it can be anything from a few hours to a couple days.
When I brew, I make starters everytime to make sure I have a good amount of yeast at the ready to start chomping on sugars. And I still can get lag times for the fermentation to start. When I brew wheats, the yeast can be very explosive, and begins anywhere from 3 to 4 hours. Other yeast strains can take 12 to 24 hours.
I have brewed my last two batches with Wyeast Rogue (Pac-Man) yeast, and made huge starters as well. Both batches took around 18 hours to start fermenting. AND, the yeast lived up to its name, but completing fermentation very quickly.
So your actual start time can vary greatly. Hang in there though, it will start.