Even at 30 days, it's extremely stressful on the team to get the system up and running, ingest our test case data, and run our tests. This is after going through this process >100 times. There's simply no way to get everything that needs to be done with an evaluation of even basic coverage in less than 30. Full stop.
Lack of adequate understanding of the potential challenges an org is going to face during implementation and use is, IMO, the single biggest reason implementations run WAY OVER budget, time, and eventually fail at integration w/the rest of the org. This is entirely ignoring the required change management, which is another beast entirely.