> If you get an extension for your tourist visa (for common-law
> status purposes in my case)
Visitor visa cannot be extended. May be you meant extension for
visitor status? If you applying for PR under in-Canada spousal /
common-law partner, then yes you can keep extending your status,
not the visa.
> are you allowed to go home for a (few) week(s). Or is the
> extension not valid anymore when you leave the country?
Extending the status won't help you to reenter Canada.
Generally, once you left the country, your visitor status in
that country will be disappear. Furthermore, if you actually
need a visa to enter Canada, you will still need a valid visa to
reenter Canada later. And having a Canadian spouse / common law
partner will make it next to impossible to get visitor visa.

Signature
(Stephan Paul) Arif Sahari Wibowo
_____ _____ _____ _____
/____ /____/ /____/ /____
_____/ / / / _____/ http://www.arifsaha.com/
Disclaimer: IANAL, IANALP, IANAMD, IANAMP, IANAAP
my statements - if any - should be treated as such.