The AirTrain schedule is on this brochure:
http://www.panynj.gov/airtrain/pdfdownload/jfk_brochure_english.pdf
I've never had to wait longer than 10 minutes. You can usually park closer to the Howard Beach station (northwestern side of the lot), compared to the Long Term Parking Lot station, if you prefer a shorter walk.
The AirTrain schedule is on