how long to wait?
last week my knee was hurting a fair amount after a ride, so I decided to take some time off. After 3 days I started riding again on the next easiest gear from what I usually do. Apparently this wasn't long enough since I started having really bad pain in it the next day at work; feels like maybe I pulled a tendon. So now I've been back off the bike for almost a week, and wondering when exactly I should start riding again; I don't want to just reinjure myself.
Ice, stretch, and see a doctor.
You will be amazed at what the first two will get you
Also, check yer bike fit and crank length.
Ditto on the bike fit
Ruling out cycling knee pain caused by a tight IT band, I'd check the bike fit. Make sure your saddle height and fore-aft adjustment is correct. Also check cleat alignment if you ride clipless pedals.
Once you're adjusted, try spinning easier gears for a while.
My deteriorated knees only feel good when I ride often. If the pain continues and you've ruled out the bike, see a doctor. Good luck.