Permalink structure change – can someone please clarify

Hello, sorry if I’m being stupid, but can someone please clarify the permalink structure change process for me?
I’ve read the codex page and understand how to change the details in the dashboard, and that the .htaccess file needs to have the mod_rewrite code in it, but I have two particular questions:

1) How long should it generally take from when I make the change in the dashboard to when the old version of the hyperlinks on bookmarking sites will re-direct to the new version of the link?
Because I did the change and waited a couple of minutes and I still got an error message when clicking on the old version of the link.

2) My .htaccess file has the mod_rewrite code, but before that it has the following line: # Switch rewrite engine off in case this was installed under HostPay. RewriteEngine Off

Is that over-writing the WP mod_rewrite code below it?

So, I suppose what I want to know is am I just not being patient enough and if I wait like half an hour after I’ve done the change, that the redirect should work?
And/or is the ‘rewrite engine off’ line overwriting the mod_rewrite code?

thanks – I appreciate the help on this!