The number you use also depends on the bobbin. My friend and myself both have bernina440 nq. And they fill a bobbin different amounts. Mine fills right to the top before stopping. Hers stops about 1/16" short of the top. Also mine works fine from a full bobbin another friend with the same machine finds she has to stop short of the top as her machine doesn't sew properly with a full bobbin. Just these 3 machines would give totally different amounts.

I use prewound NEBs (national embroidery bobbins) for machine embroidery on my domestic machine. The thread is 60 weight and the bobbins have about 131 yards of thread on them. So unless you are using Uncle Gravy's bobbin, looks like a wide range from 40 to at least 131 yards. It all depends on thread weight.

Here's another thought - I have the ability on one of my machines to adjust how full it will wind a bobbin.

I used to buy 2 500yd spools of white thread. I would keep one for the top and wind bobbins out of the other. (haven't checked this in a while) I would get almost 10 bobbins out of the 500 yd spool.

Uncle gravy, best one i've heard or seen today.

I finished a cross hatching quilt last week...used something like 7 bobbins of thread. Whew!!

I just completed FMQ a quilt that is a full size and I used 6 bobbins of aurifel thread. I didn't know you could adjust the tension when filling bobbins - must just be certain machines.

A friend of mine asked me to help him put more thread on his bobbin. I asked him when was the last time that he filled his bobbin. He said not since his wife died: almost 20 years ago.
Well, at least he does use his sewing machine for patching small stuff!

The Eldredge Two Spool uses a small size spool for the bobbin. Those hold 125 yards. Most bobbins hold about 35 yards, depending on thread size, tension and how full you fill it. Some say that not quite full bobbins sew better.

Peckish has a very good post (no. 10) that gives a lot of good information.

To fine out how many yards You are getting on a bobbin take a new spool of thread, which you know has how many yards. Start winding bobbins. When the spool is empty divide the number of bobbins into the total yardage.you will have an estimate of how many on each bobbin. Of course that will change with each different thread, but you will have an idea

Thank you UncleGravy for the laugh - everything really does depend upon the point-of-view.