The play spans roughly 3 years. Using information from the web sites linked below, I've put together a timeline for you:
February 44 BC: Caesar is named dictator perpetuus. At the Feast of Lupercalia, Antony offers him a diadem (crown), which Caesar refuses.
March 44 BC: Caesar is assassinated.
May 44 BC: Octavian meets with Marc Antony.
November 43 BC: Second Triumvirate is formed, with Octavian, Marc Antony, and Lepidus
October 42 BC: Brutus and Cassius are defeated at the battle of Philippi
The action of Julius Caesar takes place over 6 days. This parallels the actual historical time of 3 years!