First of all, I would ask what type cruises you have taken before --any 10-12-14 day cruises and if so, how did you like those? If you have and enjoyed the longer 12-14 day cruises, then most likely you would enjoy a 35 day cruise.
I would love to take a cruise around South America and don't think 30-35 days would be a bore at all. Only problem with us right now is being able to take the time to do it---but we keep it on our to do list !
Relax and enjoy--time flies when you're having fun !