Songs can be written a matter of minutes to hours. It all depends on the artist and the producer.
The best way to "control" the situation as a producer is to use the Max Martin style of production. Literally, create the hit song before the artist comes in and just have them rap over it, with pre-written choruses etc., written by you. If their lyrics suck, then you have a problem and you need to sit and coddle the artist.
Generally with a good artist, it is a seamless process. You can tell within the first 15 minutes or in a pre-production meeting if they have their shit together. If you need the money, you shouldn't care about pre-production meetings, but if you're serious, use pre-production meetings as a screening tool in order to see if the artist is in fact simply wasting your time and resources.
I've been stuck in projects because of failures to screen the artist properly. Never again will I run into the same mistakes.
Also, make sure to listen to your gut. You can generally feel a "rub" in your stomach if you and the artist aren't really clicking, but he/she is faking it because they want an "in" with you. That's why it's also usually good to party with the artist before, or have a couple drinks, just to see what they're really like. When they're drunk, they'll let their guard down and spew all types of crap. Then you'll see how conceited or humble the person is and how they really are.
Just a few tricks to help you out. But the process shouldn't take long if you have all your ducks in a row.