I’d say it strongly depends on your story, its genre and the characters themselves.
For example: If you’re writing a high school romance, I’d say 2 yrs max. between the MC and the LI. The age should also depend on whether you intend involving sexual content. In this case they would be the same age (but that’s just my personal preference!).
If they are both adults, then 4 - 6 years are fine. But to be honest, I hardly read stories where the age had a major impact on the story. So, in the end, it really depends on how much you think the age gap matters.