Correct 301 Redirect Usage?

I'm changing the file extension of my main index file from .html to .php, therefore I've chosen to use a 301 redirect for SEO purposes, so that I don't lose my current PageRank, backlinks, etc.

So in my .htaccess file I would use:

Redirect 301 /oldindex.html http://www,domain,com/newindex.php

Now, am I supposed to delete the old index file once I create the new one? OR do I keep the old index AND the new index in the same directory?

Also, how long should I keep the 301 redirect active? I've read from some people that it should be kept active forever, but wouldn't common sense tell us that it's safe to remove the 301 redirect once the search engines replace the old index with the new one?

Thanks.