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ok it varies on the presentation... you can be dealing with the classic presentation of think of a card and imagine it turning over where it should last you aloooooooongggg time. if you are not shuffling the cards it should last a good amount
I don't know how long it'll last. When I first learned the effect, I would sit there and do the effect over and over. Basically, I'd sit with one ID and one normal deck. I'd shuffle the normal deck and go through the cards one by one, doing the effect for each card I turned over. I would do this at least twice through the regular deck. I would also do it several times a day, for a couple weeks. My decks still work, but they are definitely a little 'worn.' So I'd say hundreds.
As for 2, I don't think that can be discussed without exposing the effect. There are products that can be used, and they can be found in craft stores for not much. However, I'll say this: Trying to revitalize an ID doesn't really work. Better to make or buy a whole new one.
You can touch up an older ID with a mentioned product at craft stores or a hobby store. It will run you about 5 bucks and the time to do it. Most people just fork out an additional 5 bucks and get a new ID.
It totally depends on how much you use it. I find with a moderate level of use, I can get a year to two years out of a invis. deck before it get's sloppy. For me it's worth it to just pick up a new deck when the time comes.