Usually 5 minutes after the start time on your current FP -- or 2 hours from the time you get your current FP -- whichever comes first.
You can look at the VERY bottom of your current FP - in smaller print it tells you exactly what time you can get the next one.
Of course, there are sometimes exceptions.... you might get one sooner; but never more than 2 hours from getting your current FP.
Hope that answers your question.