Quen act as a buffer for damage, and exhaust it's charge every time it deflect a blow, indeed there is no exact meters visible telling the player how much damage it can take, but it's part of the immersion, you as Geralt has to evaluate your enemies and how much damage the shield can absorb

There is a tooltop telling you how much damage can absorb in total and the calculation in combat is up tom the player, yeah, they could have added a bar to show off how much energy remain as a secondary option, but it's not that bad once you get how combat work and how much damage you can expect from a specific opponent

As for timers, yes it does pass even during the cinematic, so if a potion duration is 2 minutes and a cinematic is 30 seconds, you'll have 1:30 of timer remaining at the end of the cinematic, this includes all buffer abilities and potions

It's much more reliable to cans Quen during combat, and use longer duration potions before main story events, or just use Quen and protect your ass while drinking a potion

In comparison to the first witcher, this 3rd edition is even easier, in the first game toxicity wasn't going away when the effect of the potion was over, you had to meditate or drink white milk, making the player having to estimate potion usage more carefully

Just an aside here: there is a rune that the runemaker in HoS can craft onto your armour that automatically activates Quen every time you enter battle. I always have that one 'installed'.

Two very informative replies. Thank you.
As some enemies have been up scaled by so much in NG+ I find my main problem is the sudden loss of health.
I have not often used quen but am now having second thoughts. Time to upgrade this sign.