as a general rule i do 1.5 times the finished length...or 3 times if doubling the strings over.plus a little bit extra on the ends for finishing, amount will vary depending on how ends will be finished. below is the general rule on strings length taken from gail houghts website.
General rule for plaiting string length and string width. String length, for any number of plaiting (4,6,8,12,16), needs to be 1½ the finished length of the item being braided plus the length needed at each end, which will differ with each project. Another factor effecting string length is each braider’s individual style. If a braider tends to pull the strings outward, they will need a slightly longer length; and if a braider pulls the strings downward, forming sharper V’s, they will need slightly less length. This would probably only be apparent in a longer project.
Most projects will need an extra 6” - 12” to work with at each end. I suggest you leave a little more than that at each end until you are comfortable with how much string your style requires.
The same general rule of 1½ applies to string width. Added together, the total width of the strings needs to be 1½ times the necessary circumference of the item to be plaited. Note: this may vary depending how much the material stretches and narrows as it being braided.
Fig. 1 is a dog collar plaited in 4 plait and the extra string is needed to splice the ends in on the turn back.
Again no matter how many plait is being braided the rule is the same.
1 ½ times the desired finished length plus the length needed at each end.