We always added water to 2 cans IsoCal (and added coffee in the mornings if my husband wanted it) to the gravity bag so it was pretty thin. If the valve was opened up all the way, it would go into his stomach in as little as 10 minutes, which was way way too fast and it made him feel sick. We usually regulated the valve to a drip and he would finish in 45 min to an hour, sometime faster if he was hungry, or sometimes slower.
Gravity bags are so easy to use. We only used syringes to flush the tube with water afterwards and for feedings when we were out and about with nowhere to hang a bag.
Rinsing the bag is easy. We had a plastic hook that hung from the top of a cupboard by the sink. After rinsing, we filled the bag with clear water, hung it on the hook, and let it drain through the tube into the sink and it would be ready for the next feeding.
When we traveled, we also took a hook like that with us. Easy to hook over a door or something in hotel room when we didn't have IV pole with us.
Good luck! I loved using the gravity bags & it was less stress on Dave.
Lisa