I said before that I consider any swim over 4 miles to be a long swim for me. But here is a way to really tell if you had a long pool swim.
1. after your swim get out of the pool and take a shower
2. go home and eat a sensible if large dinner
3. Wake up at 4am to your stomach growling
4. Finally get up, take a shower, eat a large breakfast
5. Go to work (snack constantly all workday)
6. Get home by 6 and go for a little bit of a run
7. Make sure to work up a good sweat
8. Get back home, take a quick shower, and have a pre-dinner snack
If at that point you still smell like chlorine it was probably a long swim.