that’s going to vary by cooker and also ambient temp of the kitchen. The basic pressed metal type cookers will lose most heat so take longest to get to temp and have to work harder to stay there. the more insulated the cooker, the lower temp the setting and the higher the ambient temp, the faster the cooker will heat up and the less hard it will have to work to stay there. Best thing is to get an oven thermometer and check. I got a good one from Ikea, the kins with a probe and external read out, for 7 quid.