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.
Thanks guys, I ended up spending 1 day on it in the end (including the normal office interruptions, clients dropping in, phones, emails etc). I quite liked the first one, but, to give them chose went to to create a few more and somehow ended up with 8! And guess what, they chose the first one so it could have just taken me about an hour lol.
It really does depend. Ive just spent a week on an identity creating an identity palette. It really depends on the client, budget, size of the job as a whole. If an identity is being created for a complete rebranding then it can take months.