I think an earlier poster said it all, youve outgrown it when you realise that it has limitations, [b]and[/b] that they are limiting [b]you[/b].
If you havent noticed yet, then you dont [i]need [/i]anything better (although you might [i]want [/i]it)
My P bass is the best thing since sliced bread in my book, and I cannot find a single flaw on it. Doesnt mean there arent any, just that they dont limit me if there are any there.