How long do backlinks stay?

Let's say I have 100 backlinks displaying on yahoo explorer, I then change my forum profile URLs to another site, will the backlinks to my previous site eventually disappear? How long before that happens?

Tyia

They only last until the site is re-indexed. However, I've found that even after certain pages get crawled (without the link), it still shows up. Then after a month or so, it is gone.