I only started to really get into osu! around September 2012, but I only recently realized the difference between me then and the me 1 or 2 months later. I think being 'good' at the game isn't necessarily that you can play mods, and it's a totally subjective opinion (whether you're good or not).
I started off not being able to S insanes unless I was lucky to do so. A month later it clicked somewhere in my head how to play the game well and that was the point at which I started down the path of HR and DT, improving further but hitting another brick wall which I was unable to pass (I say 'was' because most of the skill I put into headbutting that wall is now gone, not because I broke past the wall).
It was after that first 'click' that I realized I'm above average - I could S insanes easily, read AR10.33 and HR insanes fairly easily (though inconsistently in the first 4-5 months).
So to not get off topic - it didn't take me a long time to get good at osu!, however the time that happens is different for everybody. You'll know when something clicks and you can S insanes easily.