The lifespan depends on the site the link is on. Pages on here for example become archived/compressed and so on the next crawl of the site, some of the links may be dropped as the spiders cant find them anymore. Similarily new links will be found on the crawl of each site.
So, it all depends on the behaviour/coding of the site. This is why getting more permanent link backs - rather than ones from forums - are best, especially on higher PR and related sites. E.g. a link from a homepage of a relevant site, with keywords in the anchor text.
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Higher Education Student Affairs