You can make it as long as you like so long as it fits your window.
How long should window valances be.
There is not an exact ratio to use for valance length but a good start is around one sixth to one quarter the height of the window.
It depends on many things but generally speaking the valance will need to be somewhere between 2 to 3 1 2 times the width of the window you re trying to cover.
If the valance is to be hung 9 feet or higher above the floor you can suitably increase the drop to up to 28 inches.
Another rule of thumb used by decorators is that the window valance depth should be 1 4 of the overall window height plus 1 inch.
Measure the width of your window opening or curtain rods and make a note.
The header style and the valance shape are other factors that can affect the drop length.
A 60 inch tall window then would call for a 16 inch deep valance while a 48 inch window calls for a valance about 13 inches deep.
Starting at the top of the window opening or curtain rod the valance should be between about 12 and 18 inches deep in most cases around 15 inches.
You may be able to make a 16 or 17 inch drop valance work in some cases but anything less than that may be too short.
However if your windows are very wide you can go a bit longer than that.
For example a fuller pinch pleat valance should have a minimum drop of 8 inches.
Then decide how long your valance will be.