I've done a LOT of shorts and, assuming it's a low/no pay gig, things are a lot more relaxed than a bigger production, so you can push things a little.
It's good to grab a script and, if they haven't shot it yet, submit a list of wild FX you think you may need to the location recordist. You may not get all or any of them, but it's always worth a shot. If there's something you think may be hard to source, such as an unusual car or something, then you should really let the director know how important it will be.
Back in film school, we were given 3-4 weeks for a short, including foley and mix and whatnot, but we were pampered. Try to get a month, if you can, but 2-3 weeks is very fair.
You should do a spotting session with the director and editor (where you get all their creative ideas, and pitch yours) once you reach picture lock, then do a runthrough with the director once you have all your elements in a rough balance. Then bring him/her in for the final mix. Some directors are keen to learn about the sound post process, but i find that showing an inexperienced director an incomplete mix isn't usually productive.
Good luck man!
Edit: One more thing; make sure your recordist gets at least 30 sec of room tone or "atmos" for each location (for each setup, if possible). That's important for stitching up gaps in your dialogue edit.