Quote:
"Nowadays, most hip implants last for 20 years or more. If you are older, your new hip may last your lifetime. If you are younger, you may need another new hip at some point"
Source:
http://www.nhs.uk/Conditions/Hip-replacement/Pages/Recovery.aspx
The information about knee replacements is vaguer:
"The knee can be replaced as often as necessary, although results tend to be slightly less effective each time. Recovery may take longer, but once you have recovered, results are usually good"
http://www.nhs.uk/Conditions/Knee-replacement/Pages/Recovery.aspx
(Click the black links at the top of those two pages for lots more information)