Sintra to Fatima is about 160km as indicated above.
For full info on public transport links between Lisbon and Fatima, see:http://bit.ly/VrhsON
The best option would probably be to get a train from Sintra to Lisbon Sete Rios train station. There are four to five an hour, See ww.cp.pt.
At Sete Rios, wander over to the Lisbon Sete Rios bus station next door and get a bus for Fatima.
For example, there is a 7h55 train from Sintra arriving at Lisbon Sete Rios at 8h33.
There is a 09:15 Rede Expressos bus from Sete Rios Bus Station, arriving Fatima Bus Station at 10:45.
The return trip is as easy with frequent services throughout the day..