Now I got one. He appeared in the same moment I started to fight the boss in the dark wood! Thanks game. This fight was epic.

Half of the troll caves have trolls inside and half has a troll outside. The outside trolls do respawn. If you find one, mark it on your map.

A larger BF is more likely to have all of the various spawn points than a smaller one. So if your BF area is tiny, look around a larger one. And use your ears. Tolls make a low groaning sound that you can often hear before you see them.

they are rare, but its there... believe

Originally posted by Jitatha:

Probably not, they are not everywhere all the time, and are quite uncommon at times. If you kill trolls in caves, those ones do not come back.
Try bigger forest, chop the trees down and mine, and maybe they will hear you.

They will respawn in caves too. It takes some time but they always respawn.
@OP, if you are hunting trolls for troll hide, be sure to mark areas with trolls on the map. If you see a troll, there must be a troll spawner there (it’s not visible but it is somewhere around) and if you kill it, another one will spawn within a couple of ingame days.

Have you looked inside troll caves? These are usually fairly common in the black forest.

I have been mining a lot. Already made it to bronze armor, because I couldn't obtain troll hides...

If you've disrupted common spots in your woods that have outside spawn spots commonly, then you'll lessen(though not completely remove) their spawning. Not all forests are the same in regards to spawn potentials either as Jitatha was eluding too.

Try going back over areas you have recently passed through. I find many times creatures spawn a lot in areas I recently occupied. A players presence will initiate creature spawning.
A lot of times I will stop, pan my camera over areas I have just travelled through, seeing deathsquitos & fulings that I know were not there a few seconds ago as I passed through. Today I came upon a Fuling garrison. I circled around it to avoid the berserkers, to start clearing out as many fulings as I could before dealing with the big guys. I then went back to the side of the camp where I made my original approach and there were now 3 lox there that were not there before.

Probably not, they are not everywhere all the time, and are quite uncommon at times. If you kill trolls in caves, those ones do not come back.
Try bigger forest, chop the trees down and mine, and maybe they will hear you.