I've experienced both ends of the spectrum at JFK. On the flip side, I had a flight last Friday night during the middle of what is usualy rush hour. We went to taxi out and ended up being #1 for take off with no delay whatsoever. It was amazing. YMMV. But if you have to get there earlier, it might be worth it to stand by for an earlier flight or do the 12 hour same day flight change.