Ehh I don't think there should really be a time limit to be honest.
From your example, you could say that the user purposefully split up the feedbacks to give him multiple feedbacks, in which that case I wouldn't really agree with it and I would remove it.
However, if lets say you sell me a bond for 1.2m or whatever, I leave you feedback, then 10 minutes later I want to buy 20m gold, I should be able to give you a feedback on both if you get what I mean.