M700 fuse has a burn rate of about 40 seconds per foot, but that varies depending on several factors, including humidity, temperature, age of fuse, and specific manufacturing issues. The only safe way to get an accurate prediction of burn rate is to test the fuse.
The official procedure for testing burn rate is to cut a 3-foot length of fuse, ignite the fuse, and note the time it takes to burn. Divide the time by three to determine the burn rate per foot. If the time is off more than + or - 5 seconds from the expected 40 seconds, perform another test to verify the results. If you repeat the test three times and your burn times vary by more than 5 seconds, do not use that time fuse, but select a different batch and try the test again.
If you have access to Army Field Manual 5-250-1, you can read more about time fuse there.
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