You would first define the tiers you need and a corresponding price.
E.g.
3 hours - 4 dollars
5 hours - 6 dollars
2 days - 20 dollars
3 days - 25 dollars
7 days - 40 dollars
And then you would create vouchers with the values you define above.
So for example when you hand someone a voucher with value 20 dollars, he'll be able to use it to get access for 2 days. Similarly if you hand someone a 4 dollar voucher, he'd get access for 3 hours.
Each voucher is valid for one device, and the timer cannot be paused by logging out.
Edit: Actually what I wrote here is wrong. You can only define pricing for 1 unit of time (1 hour, 1 week, 1 month). You then need to give someone multiple vouchers if you want them to have access for let's say 3 hours or 2 days.