From my experience, assuming you have as much information about what your client would like from the logo as you can get, you should be able to get a range of initial designs over in no more than 6 hours. Obviously amendments thereafter on-top of that - but the initial designs, no longer than 6 hours.
I preferably like to spend a day on a logo, and I tell my clients that before I start. It gives me enough time to try every option and every idea.
If it takes you longer than a day, you'd probably lose your job eventually.
EDIT: And like the poster above has said, it can also depend on your client's budget. If they're only going to pay you 50 quid, don't spend any more than 2 hours on it.