Well I use my conservatory as an office in the day and TV/music room in the evening. About 16 hours per day use in total.
Wall heater and oil filled electric rad quickly heat it up. Does get a bit hot sometimes but I paint Coolglas on the roof to keep the sun ray's out. Bit noisy in heavy rain!
There may be a question mark re planning permission if you plumb a heater into the house heating. Some stupid regulation and I may be out of date.