I mean how long do we leave it for... to get maximum effect, before we can change it to another link.
The signature link is a nice aid to getting a new website indexed. Once you are indexed, that strategy is accomplished, and you can use the four permitted links to give a boost to some of your inner pages if you wish. My personal approach is to leave my home page there permanently, plus three important inner-page links. I rotate the fourth link often like this: usually when I add a new page to the website, I place a link to it in the fourth signature link, to assist in the process of getting that page crawled for the first time.
Jim4767 gave some excellent info. Use your signature to promote different websites or inner pages if you have only one website. Promoting innerpages with links is like pepsi advertising some of their different soft drink brands. Regards
Signature links are great for faster indexing and advertisement, you can change them each time you update any page and of course if you need to advertise any site.
change them when u feel u have achieved ur target or u think that u don't want to go through with particular site page.
I agree with this. Aside from helping my site (or any page from it) to be indexed, I wait until I achieve my target/goal before changing my signature