We simply look how long people who try the game continue to play the entire 2 weeks.
I suspect the dropout rate may be dependent on the length of trial, due to the very steep learning curve. if you're just converting from another flight sim, 2 weeks will be enough for you to decide whether you like this particular flavour of game.
But it only takes a week of playing as a newb to proper flight sims (an increasing majority of potential players) to realise that you wont be even close to competitive in 2 weeks. given a month to trial it, more players might think its worth carrying on with the trial and learning to fly/gv/whatever. people are surely more likely to subscribe if they think they stand a chance of being competitive in a game?
and since the game is based on a monthly subscription it seems natural to give a months free trial.
but of course it would cost 2 weeks revenue for each new subscriber ...