Amount of use for each mask?

There isn't any way to change out the filter medium so I'd have to say they are not good for very many uses. I've used mine once and can already smell the contaminate I was trying not to breath when I have the mask on.

This is not a hazardous vapor filter, it is an odor and particulate filter and does really well for dust. If you are using them for odor control use them until the smell becomes noticeable. If you are using them for dust use them until they start to look dusty. I'm using mine while i'm using a leaf vacuum/mill and there is a lot of dust. Bottom line: Yes, you can use them several times; the only problem is that you have to treat the elastic straps GENTLY.

Last one I bought I used it for a whole season of mowing. Was getting a little dirty, but was still doing job, just a little ragged around the edges.

Depending on the type of fumes. These are disposable so I would not use one over a couple of times with minimal fumes. If you are using for epoxy - a few times is probably safe. With the porcelain refurbish, the two coats were enough as there are a lot of high fumes.