Depends on what bookmark you are making. My program tells me how many stitches each design has. If yours does not tell you, let me know which ones you want to make and if I have them, I'll look them up for you. A while back someone posted how much thread spools will stitch but I don't know about bobbin thread. I always make sure I have several ready before I start a project.

FROM EMBROIDERY LIBRARY TIPS SECTION:

Spool/Cone Size / Amount of Stitches (Needle)
5000 meter/5500 yard cone / 1,000,000 stitches
1005 meter/1100 yard spool / 200,000 stitches
201 meter/220 yard spool / 40,000 stitches

Here's a different way of looking at the equation, using a design with 5500 stitches as an example:

Needle thread used per 1,000 stitches is 181 inches/15 feet/5 yards/4.6 meters:
a. 5500 stitches/1000 stitches per 181 inches = 5.5
5.5 x 181 inches = 995.5 inches of thread needed
b. 5500 stitches/1000 stitches per 15 feet = 5.5
5.5 x 15 feet = 82.5 feet of thread needed
c. 5500 stitches/1000 stitches per 5 yards = 5.5
5.5 x 5 = 27.5 yards of thread needed
d. 5500 stitches/1000 stitches per 4.6 meters of thread = 5.5
5.5 x 4.6 = 25.3 meters of thread needed

This equation can change based on a a few factors:
1. Thread tension. If the needle thread is set loose, then more thread is used. Proper thread tension can be determined by looking at the backside of the embroidery. In a satin column, there should be 2/3 needle thread and 1/3 bobbin thread showing on the backside
2. Fabric thickness. When embroidering on thick fabric, like heavy denim or canvas, more needle thread is used
3. Stitch length. If a design uses longer stitches, then more needle thread will be consumed.

Bobbin Thread
Most pre-wound bobbins have between 114 and 150 yards of thread on them. The amount of bobbin thread varies depending on how the bobbin is wound. The "parallel" wound bobbins have more stitches per bobbin because the bobbin is wound all the way to each edge. Generally speaking, 90 inches/7.5 feet/2.5 yards/2.3 meters of bobbin thread will produce 1000 stitches.

If you have embroidery software, you might have an easy way to determine how much threaded is needed for a particular project

HOPE THIS HELPS...... *4U

Great Q!!! Here is a link from emblibrary.

http://www.emblibrary.com/EL/elprojects/SimpleProduct_ELP.aspx?CS_ProductID=PR1250&CS_Catalog=Elprojects

They also speak about bobbin thread if you go down the page. Hope this helps! *

Thanks for asking. *4u

If you can tell me, or email me a list or pics, of which bookmarks you are going to do, and if I have them in my collection I can look up the bobbin thread required in my Embird program for you. May seem like a hassle but if it's important that you know.
Or, if it helps, I usually wind two bobbins before starting FSL BM's, I figure, if I don't need the second one I'll use it for something else anyway.
Hugs n Roses, Meganne

I see you have received a couple helpful answers, but i do a lot of FSL and I always have a least (2) bobbins wound. It is always used in another project if I don't need it, or I will duplicate a design to use it up. Good stitches to you.

Great question. Thanks for asking. I too am thinking of doing FSL for Christmas presents and you have asked a question I would need answered. Thanks. Becky

One will generally do the job...depending on how large the bookmark is.