Banky's graffiti used to stick around for a while until being painted over or buffed. But now that Banksy's prints and artwork sells for vast amounts, it seems that his graffiti is being lifted from the street for "private collections" or, more likely, to be sold on eBay.
Unfortunately, the many websites and books dedicated to his work can serve as handy guidebooks for these art-snatchers.
Art of the State blog
"It would now seem that there is a market for original Banksy street art and I guess that these people are using the Banksy location maps and pictures at times published by this site and others to work out what can be lifted from the streets. It's a no-win situation. Graffiti is meant to be seen and the maps etc. help in this respect but are they now making things too easy for the scalpers? By its very nature graffiti is expected to be buffed, destroyed or gone over but soon there won't be too much to look at if this trend continues."