The length or height of the hung curtain should be to the floor or the windowsill.
Anything in between looks rather odd.
There is a trend today of puddling curtains, this is where they are made longer than the height required and “puddle” on the floor, this requires a skill in fabric selection, so that they sit well. It is used commonly in living rooms and bedrooms. s fold curtains
With puddled curtains on a hand drawn track, make sure that the width of the curtain is not excessive as they become heavy and difficult to draw back. They are also prone to picking up all the fluff from your floor, so a little bit more attention is required to keeping them looking smart.
The top of the curtain (the heading), for all curtains can be at ceiling height, above or at the top of the window frame or flush with the window reveal.
It all depends on the space that you have. Do keep your choice constant in your home as it can look very odd if some curtan heads are fixed at different heights to others, especially when you can view through from room to room or worse, in the same room.
The finished length of ceiling to floor curtains should leave a 10mm gap, so that they don’t catch the floor and are easy to draw back, the gap for finishing at a windowsill only requires 5mm.
For safety reasons, never cover radiator heaters with curtains, look for another solution, either stop the curtains short of the heaters or use a blind as an alternative style of window treatment.
Consistantancy in design of the window treatments allows for a symmetrical form to be created here in this photograph. The head of the curtains are all the same height and the window swag and tail valance details are all the same and they are all held back at the bottom of the window sill which provides a regular frame for the windows and creates a flow and rhythm to the window treatments.