Almost all my concerts have been GA concerts and (almost) all have been great experiences!
(The worst was Rihanna because of some girls thinking they are big Rihanna fans without knowing her latest album, but try to not care about stupid people like that)
Depending on the artist, if it's a popular one I come 3-12 hours (12 for the biggest ofc) before they let us in. It also depends on how close you want to be and if there is a Golden Circle/Early Entry type of thing as well, you want to come before what you think will be the moment most people arrive. (Think about when most end school/work in your area)
It also depends a lot on the arena how close you get, I was like 500 m / 0,31 miles back in line for Miley, and I got 3rd/4th row behind the Golden Circle fence because of the huge arena. I went more to the side instead of the middle where everybody else went as well. Also, keep in mind that the superfans might be getting the Golden Circle, so the people that would come there early in GA won't be taking up space in the GA line as they are in the GC one, if you get what I mean?
But yeah, if you really like the artist and wanna be close; come as early as possible. Remember to bring food and lots of water to stay hydrated, especially if it's hot weather! (Myself I'm too excited to eat on concert days, but it's still important

)