From 20c to 25c took around 1 hour to 1 1/2 hours I think. I haven't had to use it since its been warm ever since the -20c weather a while ago. My windows had seal issues which I've fixed since then so it would probably take a bit less now. Either way these heaters take a while to get warm compared to other types of heaters but they also take a while to cool off if the power goes out.