[Rickertsen2]

In the case of a vacuum system like this there are two factors you need to determine before choosing a system. They are how strong of a vacuum do you need, and in what time do you need to develope this vacuum. Once you figure out how strong the vacuum you need is, look at the datasheet for the pump system. They will usually have a table that will say "it will take X long to pump Y volume of air down to Z pressure".

If you want to determine CFM from PSI, you will need some additional piece of information. You could always buy a flow meter.