There are loads of threads about heaters giving up, mine still works fine for now but recently has been giving out hardly any heat, unless i give it some it doesnt warm up, only after about 15 mins does any form of warm air come out the vents
its a T-jet btw

If the temp gauge is in the normal position check the cabin pollen filter.
If it takes ages to get to the Normal temp position suspect either the thermostat or an air lock in the cooling system. An indicator of the latter might be a drop in the coolant level allowing air in there...