Seems like the only reason you would ever release the Eighth Gate is to use Night Moth. You're losing all your chakra either way (you've released every chakra gate; there's no way to stop all your chakra from being gone). So, the duration of Night Moth's expulsion of chakra.
I doubt there's a true chakra constraint. The description of the Gates is releasing the tenketsu that limit chakra release in the body, which everyone has.
Imagine,
- Hashirama
- A
- Others with high chakra reserves
Going into 8th gate, I'm sure they can do a lot more than what Gai did... + They can maintain it for longer time.