I've been to various minyanim in my neighborhood. I gather that a minyan is allowed to daven Ma'ariv once shki'ah has occurred.
What I have seen is that both minyanim began Mincha before shki'ah and ended after shki'ah. One minyan, consistently begins Ma'ariv immediately, while in the other, the rav gives a 5-10 minute shiur, and he almost always announces, "it's not yet time to daven Ma'ariv."
I'm not sure what he means by "not yet time", as they daven Ma'ariv between Shki'ah and tzet, which is about an hour span. But, I'm curious if there is technically any halachic requirement to wait between Mincha and Ma'ariv at all, assuming that it is the permissible time to daven Ma'ariv.