They have a right to demand monthly releases unless the dev himself has said otherwise. Doesn't matter how much time it takes for the dev to make a good update when patrons have to pay monthly. This game isn't on that level to where it's ok to pay monthly for an update that comes out every 3-4 months.
But that's not what Patreon is intended for. Patreon is a platform where you can financially support the people that you like the work from. It does not come with any guarantee that you get anything at all in return for your money. You don't "buy" anything. It's an act of faith to support someone on Patreon. So supporters do not have the right to demand anything.
And that's one of the reasons that projects on Patreon go so slowly in many cases. Plus the creator is not incentivised to deliver anything. Quite the opposite actually. If you're on Patreon you get more money if you work slowly.
For some reason I've only really seen this issue with H games on Patreon, funnily enough. Probably because that's what I see most of on Patreon, but things like hentai artists that make comics and commissions and such seem to deliver on a much more consistent basis. I guess it also has to do with the nature of game development to some extent.
I personally don't think that slow development has to be a bad thing in and of itself. The larger issue comes with PR and in being transparent with the people that support you. If you keep your supporters updated and remain honest, I look at you much more favourably than those that don't.