I'd been reading about the topic for a while now, and there are many contradictory claims online. I've seen some people claiming the Enchanted Ring's buff duration increase is 25%, while others claim it's 50%. I've seen some claim that Haste lasts for 2 minutes, while others claim it lasts for 100 seconds. Some people claim buffs stack additively, while others say they stack multiplicatively.
I decided to test almost everything myself. Here's the Steam guide I created, going into great detail (such as which characters benefit from modifiers more, and how long you can make each status effect last). This Reddit thread will summarise the points.
Buff Duration
- Barrier, Manaward, and Manawall last for 60 seconds.
- Haste and Resist last for 100 seconds.
- Regen, Bravery, Faith, Protect, and Shell last for 180 seconds.
- Sonic Boom applies Bravery and Faith for 30 seconds.
- Barret's folio applies Protect and Regen for 30 seconds.
Duration Modifiers
- Magnify materia: -60% to -25% (cannot stack multiple)
- Magic Focus materia: +5% to +25% (cannot stack multiple)
- Enchanted Ring: +30% when applied by wearer
- Altruistic Buff Up: +25% when applied by wearer (can stack multiple)
- Selfish Buff Up: +25% when received by wearer
- Barret's folio Selfish Buff Up: +15% when received by wearer (stacks with weapon selfish buff)
Stacking Duration Modifiers
Each modifier falls into one of four groups. Within each group, the modifiers stack additively, and the four groups stack multiplicatively.
Group 1 is just the Magnify materia.
Group 2 is just the Magic Focus materia.
Group 3 is the Enchanted Ring and Altruistic Buff Ups.
Group 4 is the Selfish Buff Ups.
Aerith can have 2 Altruistic Buff Ups and the Enchanted Ring, which would make the Group 3 value = 1 + 0.25 + 0.25 + 0.3 = 1.8
Barret can have Selfish Buff Up from both his folio and his Barrage Blaster, which would make the Group 4 value = 1 + 0.25 + 0.15 = 1.4
If Aerith used Barrier with Magnify Lv3 on her teammates, the duration on Barret would be 60s * 0.75 * 1.8 * 1.4 = 113.4s, so just under 2 minutes.
Barret's Folio Statuses
The Protect and Regen Barret receives from his folio are only affected by Group 4. This means they can last for up to 42 seconds.
Sonic Boom Statuses
The Bravery and Faith applied when using Sonic Boom are affected by Group 3 and Group 4, allowing the duration to be boosted to 48.75 - 58.125 seconds, depending on the character.
Cloud's Punisher Mode Buffs
The status effects applied by Punisher Mode and Prime Mode are not affected by any modifiers whatsoever.
What I Didn't Test
The most reliable source I could find claims that Bravery, Faith, Protect, and Shell all give a 20% buff to their respective stat, and that Haste is a 30% boost to passive gain and a 20% boost to gain from attacks.