Depends on the size and importance of the battle for me. If I'm just fighting a weaker lord or a bunch of bandits, I don't want to lose too many men. Getting 15 total casualties (either killed or wounded) is already huge for me in such tiny skirmishes. If I can, I try to keep it in the single-digit range, but it's sometimes inevitable especially when fighting against Rhodoks or Khergits (Rhodoks because crossbows are pretty accurate and can kill some of your men from afar even through shields, Khergits because they like to flank using their lancers).
In an important battle like a siege or field conflict of kingdom army versus kingdom army (anything above 800 total participants), I think it's normal and acceptable to get up to 120 total casualties.
However, as in everything it still depends on the situations. I will retreat at times even if I'm winning, because the prize of combat may be less than the cost of the lives of your men, such as in large battles with bandit groups (biggest group I've seen was 1,200 men), where even if you win against them, the loot is worth like only 3,000 denars in total and barely enough exp to level your men up, while you may have lost a lot of tiers 4 and up that took you a few days to train.
And there are times when your army is just outmatched, like if you get caught by a King's army or large bandit group while you have mostly lower-tier units that you were training. At that point I try to retreat as soon as I've dealt a few casualties, especially if I was able to knock the commander out from afar. But I try to stay out of such situations by bringing most of my top tier troops with me, with only one or two stacks of trainees so that I can still intimidate most lords.