I am not fond if the duration scaling used in HARP, and since it hasn't improved in the newest version, I would like to share my unofficial errata for it. The second option is based of the duration scaling table from spell-creation in College of Magic, except it has been balanced and unstupified.
These duration scaling options replaces ALL duration scaling options in existing spells, since the existing ones are both inconsistent and unbalanced, and even more often; absent.
There are two options:
Option 1:
Double duration: +2PP
Doubles duration, can be applied multiple times (x4 duration: +4PP, etc.)
Option 2:
Find the base duration in the table below, moving one step up in duration will cost +2PP, to move N steps up, costs +Nx2PP. The levels apply both to spells with fixed duration and spells with per rank duration.
Spell duration levels:
0. 1 round
1. 2 rounds
2. 5 rounds
3. 10 rounds
4. 1 minute
5. 2 minutes
6. 5 minutes
7. 10 minutes
8. 20 minutes
9. 1 hour
10. 2 hours
11. 5 hours
12. 12 hours
13. 1 day
Examples:
Bless that lasts 2 rounds/rank by default, costs, +2PP to get to 5 rounds/rank and +6PP to get to 1 minute/rank.
Calm that last 5 rounds/rank by default, costs +2PP to get to 10 rounds/rank, and +4PP to get to 1 minute/rank.
Stun Cloud that last 5 rounds by default costs +2PP to get to 10 rounds, +4PP to get to 1 minute and +22PP to make an epic whole day stun cloud.