This is one of the most common questions asked in seo, and it has an answer that many people don’t want to hear. The short answer is: “It Depends”.
There is no way to know how long a link will last, but you can probably make a good guess as to how long it will survive. If you get a link from Adobe.com, there’s a pretty good chance that as long as Adobe stays in business, your link will remain as well.
But what happens if they would get acquired by some other company? That new company might decide to keep the entire Adobe site up, or they might just redirect everything to their own site. In the second scenario, your link is gone.
Blogs – Some blogs have remained online for years, so your link may remain there just as well. But then, one day in the future, that blogger gets a deal to sell their site, and the new owner decides to “clean up” the site and dump all outbound links.
Edu sites – What used to be considered “Link Gold” in terms of getting a link from an edu to your site, today it’s not that hard to get edu links by the bundle. Smart schools have caught on to this and have taken steps to remove profiles of people who clearly aren’t students of their university.
Profile links – Easy to get, most can be automated. Do you really think these are going to last very long? It’s like anything in life. If it was easy to get, it probably isn’t worth much.
Wikis – Another source that is the current poster child for link abuse. They’re easy to set up and hardly maintained, and for some reason, Google is rewarding these links right now. But don’t expect this to last very long.
Directories – The old standby. There are very few directories that are worth getting your site listed on, but then ones that are will probably still exist ten years from now. However, there are a ton of junk directories that will fade away in time.
Social Bookmarking – With Google factoring your social presence in the rankings, any smart seo service provider is getting these links right now. There are different ways to get them, some more effective than others, and it’s just a matter of time before Google determines how to recognize the fake ones compared to the “real” links.
This is one of the reasons why no matter how well your site is ranked, you must constantly be earning more links. This is not some scam to get you to say with your seo vendor, it’s the cost of doing business and having a top ranking in Google.