Going by Wiki's and BG's tests + description, each skill over 300 (no matter if skill is capped or not) adds +1 second of duration to BP: Ward buffs, capping at 500 skill for +200 seconds (3mins 20 seconds).
This doesn't make much sense to me, because SE said "skill over cap", and 300 isn't even the cap at lv75- its just an arbitrary number.