I think it is less to do with the duration you worked with a person and more to do with how closely you worked with that person. You should analyze who can spend 'time' and write a good recommendation. All things equal, I will be inclined to choose the manager who has gone to a business school.
I am a R1 applicant for US B schools. I am in a bit of a fix about my recommendations.
I have done 2 roles primarily in my 5 year career.
1. Current role: I have been in it for 13 months. I have worked under my hiring manager for 8 months until he left for another team in the same company a few months ago. My current manager and I have worked for 5 months only. Which one of the two should I choose?
2. Previous role: I did this role for 4 years. I worked under different manager through this while but the same engineering manager, who has little experience in being a direct manager to me. I have two choices again here - the engineering manager or a direct manager during this time. Which one should I choose?
I am not stating my personal opinion here so as to not sway the opinion. It would be great if I could get a perspective on this matter from the experts..