Is 13 years of living here in America a long time?

It's 13 years. Whether it's a 'long time to have been in America' or not is entirely subjective.

Compared to a baby that was born in Nebraska this morning - it's a very long time. Compared to Mount McKinley - it's nothing.

To call it a "long time" is completely subjective. To some people a few minutes can be a long time. Whether or not you feel you've been here a long time is entirely up to you.

thats quite a long time. not amazingly long like 50 years or sumthing but still thats a long time.

In your case it's almost half your life. I lived in Italy for a little longer than that and don't consider it long enough. There were still lots of things to see and do when I retired and moved back to the US.

Yes and no