If you listen carefully you can hear when the bubble enters it's dying 5 seconds because it's ambient sound changes tone.
When you get a moment take the bastion perk off and just place the bubble in a quiet corner, then listen for the indicator of tone.
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And that is unfortunate, there is the visual cue in the middle of the bubble when it's running out, the problem with many visual cues is it allows counterplay by enemies which would nullify any advantage you get. Some people suggest using the super bar which means you can't charge the next one while it's in use. Some people are suggesting a countdown, but how and where is the question Should there be an easier way to tell when the bubble is going to pop? Perhaps only for the defender who cast it and noone else, it needs to retain some element of higher skill ceiling for play and counterplay which is why a more obvious aide is a bad thing. The subtle clues are enough and that retains skilled gameplay.
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Like I said, visual impulses like that would also help opposing guardians and defenders are the weakest subclass of the 9 in PvP. If I saw the defender bubble flash it's last 5 seconds I'd grip my shotgun then rush when it goes down, knowing that if I time it I could be sliding in just as his bubble downs and he loses his armour of light, giving me the advantage in that fight not the defender. This is why either a countdown (only for Armour of light use). On the side could work.