Recently, I was fighting an ashlander that absorbed my fatigue and caused me to pass out. I was level 30. While I was passed out because of my drained fatigue, she attacked me with a chitin axe, which barely did any damage at all. I waited for a long time and used 7 standard restore health potions while she attacked me. Eventualy, I died, while still passed out. How long does it normally last?

That depends i think on how strong this fatigue absorption was. It is not a set number but depends on the spel / effect.

Always carry some restore fatigue potions with you (as well as health and strength)

That's unusual, the longest I've been out was just a mere 5 seconds

If you pass out from fatigue damage, I'm pretty sure the amount of time it takes before you get up depends on how much your fatigue goes below zero, essentially, you'll be unconscious for however long it takes to regenerate your fatigue back to positive numbers. When I find myself in such situation, I find it helpful to drink restore fatigue potions in order to recover enough to get up more quickly, I usually keep some restore fatigue potions that I create using alchemy for this purpose.

I did that. I was at full fatigue for most of the time, but could not get up.

Is your strength down too, or do you have too much weight? Too much weight will keep you on the ground longer.

I had 5 spare pounds.