It's not the time you've spent, but the experience you gain, the knowledge you have, how FAST you can pump out quality product, the ability to make every session a success, the ability to make everything you touch in audio sound better than it should have, as well as the ability to not mess up something that is already good.
A good sign that you are no longer a newbie is when people you don't know start asking you to record them, mix them or master them because they heard your work. You KNOW you aren't a noob when people start paying you to do audio.
IMHO the jump between good hobbyist and professional is pretty huge. If you can make money off recording you're doing well.