I will be applying for an 1-year extension of my visa based on marriage tomorrow Friday, my current permitted to stay is January 3rd 2013.

I will then be given the "under consideration" stamp, lasting until Feb 3rd.

What if I would like to leave the country before then? Can I get a re-entry or do I need to wait for my extension to be approved?

Also, will I need to go on the exact date to get the final stamps, or is there a "window" (Like Feb 3rd +/- a week or similar).

Yes, get a Re Entry Permit.

You should report on the date given.

Thank you for you reply.

What if I get a multiple re-entry permit? Will that then only be valid untill the Feb 3rd (under consideration), or will it be valid for the entire year?

Only until the report back date. So a single entry is the what you need.

After you get your extension on that date you can get a multiple re-entry permit.