When properly inflated, the ring should be firm, but not hard.
The pool can only be left up year-round if the weather never dips below freezing where you live 32 degrees Fahrenheit or 0 degrees Celsius. The amount necessary will depend on sunlight, pool size, and other environmental factors, but a cup per day is a good starting point. Make sure the water is distributing evenly while filling the pool at this point. Before you set anything on the ground, clear the area of any rocks, sticks, twigs, or sharp objects that could puncture the pool. Take inventory.