Implants are a safe, well-established treatment. It's probably true to say that implants, much like natural teeth, will last for as long as you care for them. How well you look after your implants - and whether you go for your regular maintenance appointments - will have the biggest impact on how long they will last.
If you don't look after your implants they will develop a coating similar to what you get on neglected natural teeth. Left untreated, this can lead to gum infection, bleeding, soreness and general discomfort. You could get all these problems with natural teeth. If your implants are well looked after, and if the bone they are fitted to is strong and healthy, you can expect them to last for many years. However, just as with other surgical implants (such as a hip replacement) there is no lifetime guarantee.