Once my priority date for my green card comes up, how long can I stall my application?

I got approved for my green card and issued a priority date of May 5th 2008. This will come up this year. However, in 2010 I had to return to Canada to care for my elderly mother and I may not be able to get back to my job at my US employer right then. How long can I delay filing the paperwork after my priority... show more I got approved for my green card and issued a priority date of May 5th 2008. This will come up this year. However, in 2010 I had to return to Canada to care for my elderly mother and I may not be able to get back to my job at my US employer right then.

How long can I delay filing the paperwork after my priority date and not lose the green card?

In case anyone cares I worked in LA from 2001 to 2010 on TN and H-1B visas as an electronics engineer as most all the US citizen engineers that are willing to put up with the high housing prices are working in the defense industries. I already have enough years of contributions to get Social Security.
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