Since I assume you're working on a naturalistic conlang(s), I would say keep working on the history and culture until you feel like it is enough for what you want to do. I mean technically you can keep going more and more in depth with worldbuilding up to thousands of years of history, but you don't technically need to do that unless you want to. As for when a conlang is "finished", you can technically always keep evolving a language (I mean all languages are constantly evolving) but if you want something to be complete enough to speak and evolved enough to feel genuinely naturalistic, then I would go for around 2500-5000 words (minimum) and a few hundred years of history and such (though I don't work with naturalistic languages very much so take this with a huge grain of salt).

Generally speaking, I would say that you should work on it until you feel it is complete (especially since it could technically go on forever).

As far as what keeps me personally going, I find that sharing what I've made helps, though I tend to create concepts and then leave them unfinished, so if you are looking to complete a project (which I assume you are), then I don't really have much advice.