Frontline - how long till she's flea-free?

[Snoof]

Well, we've got the cat. She's gorgeous grey-and-white tux, and we've named her Sashimi. I'll be posting pictures later - I'm waiting for the camera battery to charge so I can use it, and then I'll tell you a little more about her, but suffice it to say (for now) that Hippie, who adamantly claims to be "not a cat person" is completely and utterly taken with her

The problem we're having is this - Hippie is violently allergic to fleas. Big, red, welted bumps at the site of the bite, terrible itching, and so forth. Sashimi currently has access to the back room (where her littertray and dishes are, as well as her carrier with the door taken off and her basket), the kitchen and the cupboard under the stairs, but I'm going to treat her with Frontline before I take my son to school (so her fur is dry when I return, because she does love her cuddles - she follows us around everywhere and has done since we let her out of the carrier).

We don't know that she has fleas, it's just that if she does it'll be a world of pain for Hippie if she gets them in the rest of the house, nevermind if we end up with an infestation, so we'd really rather be safe than sorry (and he got bitten by something yesterday, but he doesn't know for sure that it was a flea).

How long after treatment can we safely consider her flea-free and let her wander and explore the rest of the house? Will she need to stay where she is for another day?

It's not really an issue if she does, as she has quite a lot of space (almost as much as she had in the council flat she lived in beforehand, all told), but she's really eager to look through the rest of the house.