It is unclear whether he was referring only to Islamic State (IS) militants or also to Western-backed rebels.
Last week, 19 countries - including Mr Assad`s allies - set a deadline of 1 January for the start of peace talks.
They signed a UN statement that calls for a ceasefire to be agreed by 14 May 2016 and for free elections to be held a year later.
""Neither the Syrian government nor opposition were invited to take part in the event in Vienna so that the countries could present a unified message.
Syria `not IS incubator`
In an interview with Rai TV broadcast on Wednesday night, the Syrian president stressed that no deadline for the start of a political transition could be set while parts of the country were not controlled by his government.
"This timetable starts after starting defeating terrorism. You cannot achieve anything politically while you have the terrorists taking over many areas in Syria," he said.
"If we talk after that, one year and a half to two years is enough for any transition."
""Mr Assad has in the past used the term "terrorists" to describe both jihadist groups like IS, which controls large swathes of Syria, and Western-backed rebel forces.
The president also insisted in the interview that Syria was not an "incubator" for IS, which controls large parts of the country`s north and east.
The rise of the group, which "started in Iraq", was the result of "the support of the Turks and the Saudis and Qataris, and of course the Western policy that supported the terrorists in different ways", he said.
`Murderous dictator`
On Thursday morning, US President Barack Obama reiterated that the conflict in Syria would not end while Mr Assad remained in power.
"It is not going to be sufficient for us to have formal political processes that lead to Assad still being in power," he told reporters at the Asia-Pacific Economic Co-operation (Apec) summit in Manila.
"The reason is not simply because of my opinion of him. It is because it is unimaginable that you can stop the civil war there when the overwhelming majority of people in Syria consider him to be a brutal, murderous dictator."
But he called Russia a constructive partner in the UN-backed efforts to end the Syrian conflict, despite its opposing views on the fate of Mr Assad.
Mr Obama also warned that while the fighting continued in Syria, there was no hope of eliminating IS.
More than 250,000 people have been killed since an uprising against Mr Assad began in March 2011. Eleven million others have been forced from their homes.
