How long for SERPs to reflect changes to page

Is the change instant or does it happen over time?

[surfgatinho]

I made some smallish changes to one of my sites 3 days ago.
The Google cahed page is now showing the new version and I have crept 2 or 3 places up the SERPs.

What I'm wondering is should I make some more tweaks, assuming this is the final SERPs for my page and the 'rolling update' has finished, or do I wait and see if the page moves anymore?

Any ideas?

[Stefan]

Tweak. Serps for most pages flux often enough that it will never be "finished" anyway. If the changes you carried out seemed to improve things, I'd say assume it's done and tweak some more.

[edit_g]

When Google has cached the page, the onpage changes are reflected in the SERPs:

Message #7:
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