I believe the bobbin thread uses more than the top thread. I am currently quilting a quilt and using a different color on the top thread than the bobbin. I started out with new spools of thread for both colors and an empty bobbin to fill. I have already used up a spool of thread that I am filling bobbins with and am still using the initial top thread spool and there's quite a bit left on it. Either I was shorted on the amount of thread on the bobbin spool or the bobbin uses more thread.

My machine has the low bobbin indicator but it starts it's warning when the bobbin is still 1/3 full! I can watch through the transparent cover if I don't forget to check.

This my experience. But, I would think it would be an outcome of how the tension was set. Tight top tension, more bobbin thread used. Looser top tension, more bobbin thread used. Ya?

My new machine has a low thread indicator. When I had my old mechanical machine I did this once or twice and it seemed to work, but decided it was too much work and didn't continue. From some of the other posts, it sounds like your mileage (and tension) will vary, so experiment!

What a brilliant idea!! I'm one of those who gets lost in my thoughts and finds out that I've been sewing for "miles" without bobbin thread.:D

When my bobbin is low, it starts to hop in the bobbin case, and the stitches aren't as nice. Often, it's so bad that I end up changing it when there it still quite a bit of thread on it. As a result, I have a bunch of bobbins just under 1/4 filled.

Do you actually do this. I hope you realize that running out of spool thread is much worse than running out of bobbin thread. I seldom sew past the second patch on chain stitching before I realize I am sewing air; there is no more bobbin thread.
But the spool is thread is the one you need to watch. When that runs out, you have a tangled mess. Sometimes it takes minutes to get rid of that knot. I try to keep an eye on the spool when I know it is close to the end. If I can see the spool core through the thread, it's time to put on a new spool and wind this end of spool thread onto a bobbin. When bobbins run out, they just run out. I just deal with it.

I will only buy machines with drop in bobbins. So much easier to use and you can see how much thread is on the bobbin.

Mine does sound just a bit different. That being said sometimes when I am doing a lot of blocks , I can miss this very slight difference in sound.. and sew way to many that really aren't stitched at all. Frustrating when it happens, but I look at it as it could be worse... it could be having to un-sew all those seams.

Mine has in light that comes one when the bobbin is low, but if it gets too "lint-y" underneath, it doesn't work. If the light doesn't come on, I can tell by a tiny difference in the sound of the machine. I don't know how to explain it really, but it just sounds a bit "off". If you have a lot of sewing and the bobbins hasn't been replaced in a while, check it, or just switch to a full bobbin so you don't have to worry about it.