This is by no means a definitive answer, but I recall when I was recently looking at taking a Ryanair flight from Venice to Rome that the Ryanair bus from CIA to the Termini station reportedly took between about 48 and 60 minutes. Seems like the price was about 8-10 euros. I'd imagine you could take the Ryanair bus even if you're not flying their airline, but the Ryanair buses evidently are timed to synchronize with theri flights. So if you arrive much before the next Ryanair flight arrives, you might be looking at a city bus, which I'd assume would take longer, since it would presumably have many more stops.

It stands to reason there may also be non-Ryanair direct shuttle options, though.