For me it shows:
Total Time Spent Online: 40 minutes.
...and this is my very first post.
So you shouldn't have to have posted before it counts your online minutes.
HTH.
I just wanted to back this up, the exact same thing happened for me. I started posting rather quickly but this is true, if you lurk, you can make an account which are logged into and it is quite easy to just browse and have the time pass.