Do they fit Eb alto sax mouthpieces? What's the benefit to these?

Yes, they fit alto sax mouthpieces. The benefits to these are they both cushion teeth when playing as well as protect the mouthpiece from damage from the teeth, thus prolonging its useful life.

What is the thickness? (.4 or .8 mm etc?)

The black mouthpiece patch is 0.8mm thick and the clear mouthpiece patch is 0.35mm thick

How often should they be changed?

When the patches are worn through, then change them. Depends on your usage.

do these fit Vandoren metal mouth pieces

I have not used them on the Vandoren metal mouth piece but I suspect that it WILL fit just fine. It's slightly small on the hard rubber mouthpieces, and metal mouthpieces are usually narrower than the hard rubber, so it will probably work. They stick beautifully and it makes a huge difference in playing comfort.