Hi Adrian,
I have checked mine. It is about 35m.
Remember that the shorter, the harder it is for the glider pilot to follow, unless he is used to towing.
The lenght will also depend on the size of the field you are flying on.
Some use a flag or small parachute (I had good results with a badmington "ball") at the end, some don't, really up to you.
If you have the choice, take a bright colored line, as it will be easier to see in the air for both pilots, and also easier to find on the ground if you have release for whatever reasons.
Also remember to put a weak link, maybe about 15kgs on the glider side, and a bit more on the tug side (about 20~30 kg) as if something goes wrong, the line will break first on the glider side.