Do I have to indicate the expiration time of the cache in HTML each time?

I want cache expiration for my html page after 10 days

<META HTTP-EQUIV="expires" CONTENT="Thu, 12 Apr 2012 08:21:57 GMT">

So my question is:

• What happens after 10 days? yes, does the cache expire, but do I need to set the expiration date again?
• I have a way to set the day length in a number, for example: 10
• Do not confuse here, please give me a link.

Please, help...