How long do you GM's generally give your players (in days) to prepare for the run?

Varies from run to run. Some things are "your window of opportunity closes in an hour, so plan fast", and others allow for days, or even weeks.

Players? Typical week-long pauses between sessions.
Characters? Typically several days, 3-7. Otherwise it's all Probing Targets and no fun.

Game time - a few days to one week, baring rush jobs.

Real time - no more than two hours for a typical run, never more than four hours. That's roughly half a game session for my group. That's pure planning time after legwork and recon.

I'm flexible with plans though. I'm aware of the fact that the players had less real planning time than their characters, so if they forgot something minor, say an extra piece of gear or an extra piece of legwork, I allow them to modify their plan on the fly. This actually comes up very rarely, but I leave the option open.

It really depends on the job. I've been running my group through both Missions and Ghost Cartels so the time they have to plan is limited in many cases. When its a run I've created it can be anywhere from a few hours to a week or longer. This is game time.
For the players to plan something, if they don't get something off the ground in a few hours I start making things happen. I've had too many sessions wasted with planning that went no where. I'd rather have a bad plan set up and ready to go than a good plan that took the whole session to come up with leaving no game time to implement. This seems to happen more because a few players I've had are worried about making the 'right' choice rather than any choice.
If you want to set a time limit for your own players, make it no more than half the time you have allocated to the game session. That way they have time to get it to work too. Assuming of course you do one run/adventure/job/whatever per game session.

5 seconds. I have two players who will, literally, spend 6 months planning if you give them 10 minutes of game time, and four players who want to actually, you know, run (and the two who plan also admit they spend too much time planning). So now everything keeps asploding on them so they never have time to overplan

I plan my runs so they can be completed in one session. I've never liked the "cliffhanger" philosphy of game planning and it just leaves things feeling unsatisfied at the end.

Because of this my games feel very episodic, which my characters like. I will plan plots that will span several sessions, but I always try to have the action come to a close at the end of a particular session.

Our sessions tend to be about 6 hours long with about 4 hours of actual play time after eating, socializing, and kid-in-bed-putting is all said and done.