I should have said: "don't spend your stamina on Quen during combat". It's fine to apply Quen before you start a fight. As stamina regenerates quickly when you are not in combat yet.
And of course when you spend talent-points into one sign, but not another, it changes the equation.
My point was just: don't expect Quen to protect you while you slowly kill enemies. You need to be quick, and kill enemies before they kill you. As a rule of thumb. And of course, you should try not to get hit while fighting (positioning, singling out enemies, etc).
And as I said, I'm only level 6 or 7 at the moment. So what do I know ? My comment are based on experience from TW1 and TW2, other games, and a little bit TW3.
Oh, got another tip:
8) When fighting a pack of humans, go kill the "ranged" first (the ones who shoot with a bow at you). They usually have little health and armor, and go down in 1-2 hits. If you leave those for last, they will do a large amount of damage to you during the whole fight. Take em out first. Then take out the "light enemies", the ones that you can kill with fast attacks. Leave the "heavy" enemies for last. The ones that have shields, the ones that have axes, and that require you to do strong attacks. When fighting bosses, usually take out the adds first, and the boss last. (Although there are probably exceptions, where the adds are invulnerable, or respawn, until you kill the boss).