I'm a Champion, and I've been playing the game since October 16, 2011. I stopped playing just before missions were introduced, but came back partway through last year and got to Defender - probably took a few weeks. I came back in early September this year and tried ridiculously to get to Champion - I needed to collect 12 Eternal Orbs, after all - and got there mid-last month, I think. So it took me about two and a half months to get there?
I've paid for energy and stuff a couple times, but nothing substantial or common. I mostly stick with F2P. I'm currently on the mission The Silent Legion, which I'm too scared to try.
Also, you can't "finish" the game. It's an MMO. It has no finish.
I've been playing since the days of the beta, though I've had a lot of on-and-off again bursts of play. I currently have about 17 days of playtime on my main character, and almost 500 hours since the Steam release. Over the years, I've purchased energy once, an elevator pass, and the expansion. I've had jobs and gone to school, so I think it took me longer than most to get to end game. I reached Vanguard around June of this year, just before the BS update and changing the craft system. This game has come a long way from what it once was.