depends on the thread - knitting threads are commonly 50 or 100 metres long, however if it's a stranded embroidery cotton, it's the same length as stranded silk, 8 metres, & good old sewing thread comes in spools of 100 or 500 metres.

just depends

oh, Hrothgar mentioned tapestry & those threads normally come in hanks 13.7metres/15yards long

Well is where you get water, as my old friend Millie used to say. He would also finish every sentance with "well", so naturally we called him Millie-well.