How many feet of fabric are there in a bolt of cloth?

Variously 30 yards, 28 ells or 40 feet according to the OED.

I have it as 40 yards = 120 feet.
A bolt is 32 'ELLs' , an Ell being 1 and 1/4 yards (or 45 inches)

'The Economist Desk Companion' (published 1992) says that the length of a bolt is '100 yards (on modern looms)'. The width is said to be 42 inches for cotton and 60 inches for wool.

Chambers chickens out and just defines it as 'a roll of a defiite measure (of cloth, etc)'

This question seems to be a variant on 'how long is a piece of string?'

I for a long time did inventory for a living. I counted bolts of fabric that were 100 yards so they were 300 feet

Having grown up working in a fabric store, I can definitely say that a "Bolt" of fabric is 'a roll of definite measurement'.

Polycotton, rayon, other synthetics, light cotton, heavy cotton, vinyl, fleece, and so on: Due to the thickness of the fabric, a bolt is typically 'full' when it's within a certain circumference range so that it fits neatly in storage or on the shelf. Then these are standardized for that manufacturer and fabric (all fleece coming from a certain manufacturer were 30 yards, for example, while another manufacturer might fit 40 yards if their material was thinner)

Width also played a factor, and varied almost as much.

I will say that the greatest length of the ones we carried were 120 yards, and I cannot remember the shortest, but 30, 45, and 100 were common.