What is a "rental bus"? A shuttle bus to a rental car company? They circulate ocntinuously all the time, so no need to worry about how long it'll take you. If it's some sort of bus you need to meet at a specific time, you'd probably want to phone by cell phone when you're through Customs.

Immigration is often the largest delay, followed by wating for your bags. Actually going through Customs is usually just a matter of handing over the form you got on the flight and had initialed at Immigration. Unless you get pulled aside for secondary inspection, that should take a minute. The time to get through Immigration is, as noted here on a regular basis, dependent on how many other international flights arrive at the same terminal at the same time.