We went in January (so a pretty slow time) and did not have FOTL.
We arrived at US right at opening and did all of the rides, including stuff I couldn't care less about like Jimmy Neutron and Shrek. We also were able to do the Mummy and MIB 3-4 times each. We did not get to do Disater as it wasn't open yet, and wouldn't open until the afternoon.
Then we walked over to IOA and had lunch. The weather turned really crappy and lightning closed pretty much every ride for almost 3 hours. We did eventually get to ride Hulk (1 time), the Dragons (about 4 times each), Spiderman (3 times), the Spin-y ride (X-Force or something), and Cat in the Hat. Due to the rain, and the fact we hate water rides, we didn't do JP or the other 2 water rides, but we totally could have had time to. I think we left a good 1.5 hours before park closing.
That being said, we had been to IOA many times before (US was new to us), so we didn't feel the need to explore every nook and crany. On a slow day (or with FOTL), you could do both, no problem!