Bulwark's extra duration seems to be a minimum of +1 game tick per rank. The only shield tier where the increase in duration is larger than +1 per rank is tier 90 (and only for rank 3). This does then mean that using debilitate without a shield will still see an increase in duration from the Bulwark perk for all ranks.
Bulwark increases Debilitate's uptime by 2.4 seconds out of 30 if used with a tier 90 shield. This is a 17.3% increase, resulting in 8.65% overall increased damage reduction provided the player uses the ability every 30 seconds when it comes off cooldown, has 100% hit chance, is fighting against one opponent, and damage taken is not classified as hard typeless.