I have a square template and cut one out of linen and then wash and dry it on the highest setting then measure it. Note the new measurement, then wash it again and remeasure. If it has shrinkage then, I rewash it. If not, 2 was fine.
If it wasn't measured, it depends. If it's handkerchief linen, or very light weight, I'd do a third to be safe. I find that linen weight usually does have a small amount of shrinkage on the third, but usually will stop or be minimal after that. I also don't wash the garments the same as I do the sample, which helps as well. Medium weight and heavy seems to not shrink after 2 as much, although it still can.
Dyes sometimes seem to affect it, too, at least what I buy in the States. Natural and bleached shrinks less then colored and brightly colored shrinks slightly more then all of them.
However, I plan for the top loss of 15% when sewing with linen and use slightly smaller seam allowances then a commercial pattern calls for or if drafting my own, add about 1/8" or 3mm to the pattern.