not a free trial, since you buy MOP, you unlock MOP on your account, there is no reason to lock you based on level or gear you get during that time. once time's up, your account freeze with whatever level/gear/achiev you gain, as usual.
Just a reasonable amount of time to play the product you bought. after that, want more time, buy it, but at very least, you'd buy MOP with knowledge you will be able to play some without additional fee. i think that's a fair business model and a right way to treat your consumer base.