It depends on what time you get there. If you get to PFF right when it opens, it'll probably be about a 45 minute wait. I went mid-August and got there around 9:15am and waited for it to open at 10am and was 1 of the first 10 people in line. By the time they dropped the line, the line was pretty long already.
And if you going in the middle of the day, expect at least a 2 hour wait. Your best bet would be to go there early and wait for it to open.
As for Pixie Hollow, the line is usually a lot shorter. The usual wait time was about 35 minutes when I went although I've seen it up to 55 minutes long.
I just returned from a trip today and only waited less than 15 minutes for Pixie Hollow. My daughter was so happy! Just 1/2 an hour earlier it was almost twice that. I kept walking by waiting for a short wait time all the days I was there. We never did go to PFF so I have don't have any info on wait times for that one.
Have a fun trip!!
Just realized that you said you'll be going after Labor Day and not on Labor Day. I would suspect that the wait times will be a lot shorter since summer is over and all the kids are back in school already. Perhaps someone who went recently will be able to give you a better answer...