It depends on the thread size, the pattern, the technique, and the tension.
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Shown here are one yard samples of 3 different sizes of DMC Cordonnet Special. The loose thread dangling in the larger photo is part of the total 1 yard length, since I needed some out to do the tatting with. The pattern is the start of the popular edging, "Hen and Chicks" which is found in many pattern books.
The green on top is done in size 20, the white in the middle is size 50, and the pinkish one on the bottom is size 80. As you can see, with a smaller size thread a bit more of the pattern can be done before running out.
For simplicity's sake, let's just say that I am able to tat about a half inch with 1 yard. Actually, I only used 26 inches because the 10 inches extra were the ends sticking out. So a full inch of a narrow tatted edging would need about 52 inches of thread. If I wanted 10 inches of edging, I'd need 15 yards, and so on.
This is just a rough guide. If you have a large project planned, it's best to measure some thread, maybe several yards of the size you plan to use. Then work part of your pattern -- a motif, a repeat, or some part of it -- then measure how much thread you have left. The difference is how much you used for that much of the pattern.
If you then do a little math, you'll be able to come up with a close estimate of how much thread you'll need for your project. (It's best to buy a bit extra just in case!)