https://www.sciencedaily.com/releases/2018/10/181026102619.htm

Researchers at the University of East Anglia (UEA) found the length of time between product or service consumption and posting affects the review given.

For example, the sooner customers post a review the more they 'zoom-in' on their experience, focusing on the more concrete aspects, even on small details, and the more negative they are. As the time increases, reviewers 'zoom-out' and have a more positive view, focussing instead on the general experience and more abstract aspects.