It's around 7-8 cm. I usually add a folded wool blanket on my side. This setup is very comfortable for side sleeping and back sleeping.

I also found it key to use a small buckwheat pillow.

If you side sleep, a very good alternative and cheap setup is a buckwheat mattress.

I'm quite light (~75 kg) so heavier people should probably have a few extra cm. Wool compresses a lot and requires periodic shaking to keep shape.