I think length of time depends on what condition is being treated. Spot has chronic pancreatitis - which comes and goes - so I give it to him daily during episodes, but he can go weeks without in between it if he's feeling ok. I think certain conditions that are always present, like CKD, might be different, and require it daily forever. If it is a condition that isn't always present, you may be able to get away with giving fluids only during times of episodes... it really just depends on what he needs. You can tell when a cat is dehydrated by pulling up the skin on the scruff of their neck and releasing it. If it doesn't immediately snap back when released, kitty probably needs some extra hydration.
You can warm the fluids by letting it sit in a bowl of warm water for a few minutes.