dragn, I don't know exactly how long, but it started yesterday, and finished today, and probably over 24 hrs. I cooked it on an induction burner, and had it sort of foaming up on 3, so I turned it to 2, and occasionally would stir it with a silicone spatula as I was nearby, but it never formed any layer on the bottom. Eventually, it started getting thicker (fortunately, I have a very large splatter cover for this!), so I turned it to 1, and about midnight I turned it off, put the glass cover on, and turned it back on about 7 am (still warm then - I wasn't worried about leaving concentrated tomatoes out this long, with the acid in them), and this evening, I turned it up to 5, and stirred it several minutes, like I would when cooking down a chile paste, and eventually got it thick enough.
I have made paste much faster, by adding some dried tomatoes to the liquefied tomatoes, to thicken them much more to start out.