how long should a scarf be?

[ZombiesAtYourCervix]

a knitted one? i don't appear to be able to stop.

At least long enough to go round your neck once.
Ideally the same as your span/height (the same unless you have odd proportions)
If I'm making it - usually until I either a-get really bored or b-run out of yarn.
Dr Who scarves are fab, I just wish I had the staying power!

I can only knit scarves and my rule is, as tall as the person going to wear it. Remember that can include tassels, and then once worn it will stretch (some times by heaps!)

Depends really. I quite like neatish ones where you just cross over at the front and fasten with a brooch or shawl pin and they aren't very long at all.

Otherwise I have a longish one that goes twice round my neck with enough to tie at the front so probably about 150cm

For one that isn't specifically patterned to have a beginning middle and end,iyswim, I just keep going until the yarn runs out.