Actually...the minimal guard duration is 10 seconds. The time it stays up the 1:1 with wield rate and boosted in 50% if you have self-preservation. With round shields i have shield up for 28 seconds when i use guard or ant defensive shield action...but you need to understand the mechanics balance...you cannot expect to keep the 99% block rate and still have it up full time. You need to see that its like a libra...when wield rate goes up the block rate goes down and vice-versa. It was meant to players actually find their optimal balance between these 2 attributes, using what fit them better...but instead everyone want the 99% block rate of scutum and shields up full time...and this won't happen.
IMO, once again people asked the wrong thing, the dev team used it as base for changes and, when the change is made, people will say that it wasn't what they wanted and regret it because GLA will be even weaker defensively.