how much can i raise my stem? I've got one of these stems: 



It's currently fully inserted into the bike, and it's too low for me.

Can I simply loosen it, raise it 5cm and tighten it again?

What's the minimum length that should be inserted? will there be markings like on the seatpost?

The stem is Aluminum. The frame is Steel.
With most older quill stems, there must be a minimum of one inch of the stem (above the binder wedge) inside the frame for safety. 

Most stems have marks on them similar to seat posts, yes. The longer the quill, the more of the stem needs to be in the frame. I suggest 25% of the length as a minimum.

The scratch marks on the stem in the photo you linked show about where that one should be inserted as a minimum. 

Yes, you can loosen the bolt and slide the stem farther up, to raise it, with this style stem. How much you can raise it varies by model and your frame.