Originally Posted by AutoSlash

It depends on the situation. Typically, a monthly rental is considered anything more than 27 days, so when we look for deals, we typically check durations of 27, 28, 29, 30, and 31 days to see what comes out most cost-effective. Sometimes it's less expensive to rent for 27 days since you're on a weekly rate plan, while other times it's better to rent for 28-31 days. Sometimes, the price can be the same (or only marginally more) to rent for 30 or 31 days than for 28 days, since you're paying for the whole month already. Again, YMMW and it depends on the company, location, etc.