My DD was exactly the same at that age - after *exactly* one hour she would clearly let me know she was tired and it would be relatively easy to settle her to sleep.. Anyone I would mention this to (as I was also worried she wasn't getting enough stimulation in her limited "up" time) would laugh and tell me how lucky I was!!! Even now at 11 months she tends to get tired after about 2-3 hours.. As other have said, just go with the flow...