I would advise looking at the industry and the nature of the collection period, and why it's longer/shorter. Some business models, for example the ones that gets paid via insurance companies, the collection periods are naturally longer. Some are much shorter. When working with the seller and in closing the deal, you want to build that relationship and create terms fair to the business model, rather than having hard rules (there are of course exceptions to this depending on the industries and your specific thesis etc.). One way of doing this is analyzing the aging accounts and historical collection percentages broken up in different time brackets e.g. 30, 60, 90 and 120 days to estimate the likelihood of collection.

A lot of good responses already. It depends on the business and nature of those receivables. The more information you can get on the receivables outstanding for more than 90 days, will help inform your decision on how to treat during diligence and post-closing.