Sometimes I'll read a message via my mobile phone, but wait until I get home to the pc before I type a proper reply. If I have a long booking, am travelling or simply having a day off that might not be for 24 hours, I may also not be quite sure of other arrangements that day (I have sometimes jumped in, to then find other bookings change in the meantime) and like to wait a little before giving a definite respons. Some guys do expect a reply within fairly quickly though. I'd advise waiting a short while before following up, if she doesn't reply by the end of the day contact someone else.
Or you could just phone her.