The normal lifespan of US Currency is as follows...
Denomination....... Life Span
$ 1 ...............21 months
$ 5 ..............16 months
$ 10 .............18 months
$ 20 .............24 months
$ 50 .............55 months
$100 .............89 months
The notes are "Redesigned" or a new series (or signatures) every few years. New people take office or the note is redesigned giving it a new look such as you have seen in the past 10 years for our currency. The BEP is very resistant to changing the 'look' of US currency as it is the most recognizable currency in the world and its design has been a trusted look and feel. Because of counterfeiting the BEP has been forced to add security features and change the design and overall appearance (and plans to continue to do so in the future).

As far as foreign currencies they too have been plagued by the same problems the US faces and must change their currency periodically as well. Other factors include governments changing, merging with other countries (like the Euro, WAS, CAS, etc), inflation (like Zimbabwe), just to name a few. Each quarterly issue of the IBNS magazine features a long list of new issues being released. So, other countries issue new designs probably more often than the US. They also experience similar life spans of notes.