Upvote:1
Your husband can apply up to 3 months before his intended date of travel.
No-one can predict his chances of success. Your PhD viva and graduation is a reasonable premise for his visit, but IMHO it is unlikely to be sufficient to justify a 5 month visit. Planning to be away from his business for so long and being unable to cover the costs of the trip himself is also a red flag.
If you decide to apply I suggest you shorten the intended length of visit to the say 2-3 weeks immediately around your graduation.