I think they should do something like 7 days from purchase / 1 hour of usage - whichever comes first. More or less like in movie rentals that have 30 days/24hours. This way it would be harder for people to abuse. This could be the standard for all countries.
Thats the best idea. 1 hour of actual usage or 7 days of ownership, whatever comes first. I like that idea.
I usually can decide within 15min if an application is useful for me or not (most garbage I delete within 1 minute). But I often download apps when I read a nice review, but don't start them until the evening or the next day when I have some downtime. The policy you suggest would allow me to download and test a day or two later and still had the benefit (for the developer) that you don't use it more than one hour.
Maybe the 'trial' time should be adjusted by 'price' so that more expensive pro apps can be tested more throughly ( 99c = 10min, $1.99=15min .... )