Your spouse cannot be on welfare at the time the application to sponsor you is submitted and also cannot go on welfare at any point during the processing of the application.
If your spouse is on welfare now, your spouse should fine a way to get themselves off welfare and start working, and only then submit the application to sponsor you for PR.
If your sponsor is on welfare when the application is received or goes on welfare while the application is being processed, this will result in the application being refused.
if other criteria met, any time. Need documentary proof (marriage certificate). It may make sense for some to wait a bit longer to show relationship but that's not the same question as being allowed to and being eligible.