Once a day. Yes, some people like to feed hatchlings more often. I tend to feed enough that my tort doesn't finish it all on most days, so some might still be left for an afternoon snack if he wants. I'd rather feed less and feed daily if the tort was on a diet than skip days (some people say to skip days so your tort doesn't get overweight like captive torts tend to do). Tracking your tort's weight and SCL weekly for hatchlings and monthly for older torts is a great idea. Some species have growth charts available online.
I've always fed Nelson once a day, even when he was a hatchling.
There were times he wouldn't finish his meal in one "sitting" and he would go back later and finish it. Now he's a little piggy and always finishes his meals