>From Tiburtina station to the airport there's a train every 15 minutes. It is not direct but with a few stops in between, it takes about 45 minute and costs around 4 EUR. So, you should be able to get to the airport around 8am, just in time for the 2 hours advance check in. On the other hand, if the train from Venice is late or if you travel on peak season and there's overbooking you might risk...

Ciao Flavio in Rome