Based on my own experience, officer is more interested in evaluating your financial capacity rather your parents’. As far as timing is concerned, you should mention more than 6 months and provide valid reason like spend quality time with you. When you write invitation letter, I would advise to do emotional touch as well to show that you are willing to take financial burden when they arrive which ultimately answers ties to home country and canada question.
Providing investment info, property info or Insurance isn’t enough to assure that they will go back after authorized stat in Canada. You have to talk bit more about how your parents are socially involved with family and all to answer all questions.
