Since all the wikis are horribly outdated or just outright wrong in lots of its information I decided to do some reasearch myself and here's the results. Also, because I was really bored.
Note: When I say "all BPWs" I generally mean all defensive BPWs, except for Glittering Ruby, Earthen Ward and Aerial Armor. Special rules apply to those, see below for information on them.
- All BPWs have a base duration of tier*1min. The timer starts at activation of the BP (when the timer resets).
- All BPWs are tiered from 1-3. As stated above, each tier adds one minute to the base duration
- All BPWs add one second per skill level over 300 to their base duration
- Duration bonus from skill caps at 500 skill (200 extra seconds or 3:20)
- All BPWs overwrite themselves, even with much less skill, resulting in the buff wearing off before it would have if it was not overwritten
- RDM Enspell II will take priority over BPW Enspell, which take priority over RDM Enspell I.
- Spikes BPWs have the same priority as BLM Spikes, they will both overwrite each other (unlike with Enspells). The potency does not matter (usually higher from BPWs), nor does skill.
List of BP tiers:
Tier 1: Crimson Howl, Inferno Howl, Earthen Armor
Tier 2: Fleet Wind, Rolling Thunder
Tier 3: Shining Ruby, Ecliptic Growl, Ecliptic Howl, Hastega, Frost Armor, Lightning Armor, Noctoshield, Dream Shroud
Exceptions
Earthen Ward & Aerial Armor
Last for 15min, independant of skill.
Glittering Ruby
Skill doesn't seem to affect it at all. The duration is directly dependant on the attribute bonus, as it decays from the original bonus value to 0, at which point the effect wears off. It decreases by 1 attribute level with each healing tick (10s). So the duration is approximately bonus*10s, depending on when the healing tick occurs (which, in turn, depends on when you enter a zone).
Potency data
Glittering Ruby
The attribute that is enhanced is picked at random for each player that gets hit with it. It's possible to receive the same buff that's already active (if a second GR is cast or if you recast it when the timer is back up). It will overwrite itself only with a higher potency, though. At this point I'm not yet sure whether that's maximum potency or current potency (as it decays), but shouldn't be hard to find out from further testing.
Concerning the stat bonus itself, it seems there's a correlation to Carbuncle's perpetuation cost (seems to be roughly twice the perpetuation cost, or more accurately, perpetuation cost seems to be half of it). However, due to lacking data for certain levels (hard to find people to sync to all levels between 44 and 99... test server would probably help, but having trouble installing it) and due to uncertainty in the validity of current perpetuation cost information I can't draw an exact conclusion on this. Wouldn't turn down any further insight into this, if someone can be bothered to post their data. Here's my data, gathered mostly through level sync:
[level - potency - perpetuation cost]
Lv.49 - 12 - 6 MP/tick
Lv.54 - 13 - 7 MP/tick
Lv.60 - 15 - 7 MP/tick
Lv.61 - 15 - 7 MP/tick
Lv.65 - 16 - 8 MP/tick
Lv.66 - 16 - 8 MP/tick
Lv.67 - 16 - 8 MP/tick
Lv.70 - 17 - 8 MP/tick
Lv.71 - 17 - 8 MP/tick
Lv.75 - 18 - 9 MP/tick
Lv.76 - 18 - 9 MP/tick
Lv.79 - 18 - 9 MP/tick
Lv.92 - 21 - 11 MP/tick
Lv.94 - 21 - 11 MP/tick
Lv.95 - 21 - 11 MP/tick
Lv.96 - 22 - 11 MP/tick
Lv.99 - 22 - 11 MP/tick
If someone could complete this for the missing levels (especially Lv44) I'd be very happy to see it.
Earthen Ward
The currently listed formula for its potency (2*level +50) still worked at 99 (reduced 248 damage from 1000 Needles), so one can reasonably assume that it hasn't changed.
Aerial Armor
I very rarely get less than three shadows, even when I level sync to lower levels. I'm really not sure what it depends on, or why it's variable in the first place.
Noctoshield
Flat 13 damage reduction, independant of skill or level. Is overwritten by and does not overwrite both Phalanx and Phalanx II.