Firstly, what does "complete" mean to me. Well, "complete" means getting 100%, while "finish" means seeing the credits roll. There are a few exceptions to that though.
For example, when the game pulls a fake credits halfway through the game in an attempt to make you think the second half of the game is just "post-game content". In such a scenario, I don't consider it finished until you've seen the real ending of the game.
Another example of an exception is when the game has a ridiculous amount of meaningless busywork. I mean if you've seen and done everything, with the only thing being left that you need to kill 10,000 of each monster for an achievement or something, you've pretty much completed it.
Mind you, I don't adhere strictly to my own definitions. It's too much effort always having to distinguish between the two. Besides, for most people, the two terms are completely interchangeable anyway.
Now as for how many games I tend to finish. Honestly, not that many.
Definitely more now than when I was younger. Most recently I've been going through the games I abandoned, and finishing them off. So this year alone, I've probably finished close to ten games already.
But I've always been more of a "move onto the next game when you get bored" kind of guy. Although considering I'm finishing more than when I was younger, I suppose my attention span must be growing.
Now as for how long it takes me to finish a game. Usually a month or two.
I mean, most games seem to be around 30 hours. And I'll usually spend an hour or two playing it on most days. It adds up quickly. So that's like what... maybe ten a year?
As for getting 100% completion. I don't really aim for that unless it's a game I really REALLY like, or getting 100% is really REALLY easy.
I just don't find it fun to do busywork. Which is kind of odd for me, considering I do find it fun to grind. I'm not sure what the difference is, but apparently there must be one.