I have a super-long one that I mistakenly ordered somewhere. I swear it is like 30m. I only ever use 1/4 of it, and it's super-prone to tangling. I have no idea why anyone would want a longe line that long, and you can't even hold it properly.
I have a lunge line from Dover that is a 10 meter length, which gives a 20 meter circle for the horse. It also has little indicators sewn on it, so that you can shorten the line and have the horse do a 15 meter circle or a 10 meter circle. It is quite handy to be able to help one 'place' the circles. I am not sure I could use one that was much longer unless I had a horse that was very easy to lunge, or a long lunge whip, or both.
I once saw a woman using a fly fishing rod and reel with fairly large fishing line as her lunge whip. Her mare knew exactly how long a regular lunge whip was, and cheated like mad.