It'd probably be helpful if the store let the developer set the trial period. An appropriately set period is going to vary by what the software does and the devs/publisher are the most likely to have (a) the necessary insight involved and (b) the highest incentive to get it right.
I heard people would buy an app, "backed it up" with Titanium backup, get a refund and restore. Not sure if they put something into Android/Play now to counter that though...
It could make more sense to leave the period up to the developer to decide (with a few options to choose from).