Validity of OINP

Hi!

Tldr What is the validity of the OINP?

Details I have a situation where I need to travel to my home country for 2 months in Dec-Jan but would still like to start the PR process before it.

I intend to decline the ITA before I come back so that I can apply for PCC from outside my home country for PR process.

I was wondering if I get OINP then would I be able to reject the ITA before I come back to Canada. Basically how long can I reject the ITAs?