We booked our overnight train from Rome to Palermo at the weekend, departing at 20:15 in June. How far in advance of departure should we go to the train station? Thank you!

Emily, 

I took the Sleeper from Milano to Messina.  I was able to board about thirty minutes before departure.  I would suggest you go to Roma Termini and go to the ticket office and ask what time you will be able to board.  The train originates there so it should be on the Binaro early.  They should have the information you require.