Our intention was to apply right after we got married but we had to wait for the marriage certificate then life happened then we had to wait for the NOA Option C, then life happened again. We got married 31 August 2015 but did not submit the application until March 2016.
I know a couple that has been married for 10 years and still has not done it yet because his job is out of Canada.
There really is no time limit.