I've been reusing mine everyday for about 2 weeks. I've been having them on all day. I guess it depends on how often and how long you wear them for, but you can definitely make them last a long time!
I've used my 3 times already...going on 4 tonight.
I don't know that in terms of eye hygiene it is a great idea to reuse them--eyes are sensitive, and these will definitely collect germs because of their handling and their sticky backs. I think if you reused these a lot or shared them between people you'd be asking for an eye infection. However, you definitely CAN reuse them--I find that the sticker lasts for awhile, and the included glue means you can always add some adhesive.