Most of the weaving I do involves putting a 12 to 15 yard warp on the loom and then weaving it off. This means that a run of kitchen towels will usually include 11 to 14 towels and a run of linen face cloths could be as many as 30.
There are two ways of dealing with the huge length of soon-to-be fabric. You can start at the beginning and weave along until you get to the end of the warp. I've done this several times. Each towel is exactly the same. Once you get the fabric off the loom and washed you can decide how many towels you want and therefore how long each one will be.
The other way to weave is to measure as you go. You can make each towel different is some way when you know where it begins and ends. You can add horizontal stripes and know that each towel will look the same. Or put them in different spots if you want each to be different. You can weave the hems using a different pattern or yarn as long as you know where the hem begins and ends.
So how to you measure each towel? I like pinning a tape measure to the side of my weaving. It doesn't get in the way and I can see where I am in the process. Is it time for the next horizontal stripe? Do I want to weave the next towel in blue? Is it time to start the hem? Everything is doable.
Unfortunately, they don't make cloth tape measures anymore. The thin plastic ones work pretty well but they end up rough and scratchy from all the pinholes. My daughter found me some cotton fabric tape that looks similar to a tape measure but it isn't accurate enough to measure anything with. And it only goes to 10" and then repeats. It's for decoration purposes only. Well, that's OK.
I have a cotton tape "measure" that I have measured off to be the correct length for each of the projects I commonly weave. The one for linen face cloths is 18" long. It looks like 19" on my decorative tape because that's how far off the measurement it is. I could do the same thing by measuring a length of yarn or cording to the 18" but I like knowing where I am in the process. Close to the beginning? Close to the end? Halfway? I can figure all of that out even if I'm using a tape that isn't an accurate measure.