"I don't actually think we will necessarily need to spend a full two years, but let's see how we go", Johnson said. "We will invoke that".
"I find some of the take-or-leave it bravado we hear from those who assume Europe has no option but to give us everything we want more than a little naïve", he told the Chicago Council on Global Affairs.
Mr Johnson also revealed more details about the Government's Brexit plan and hinted there could be free-movement sweeteners offered to non-EU countries as part of any trade deal, such as more visas and work permits.
May previously said she would not trigger Article 50 this year and refused to say when she will begin exit proceedings with trading block, to the dismay of some.
David Davis fell foul of Downing Street earlier this month when he pointed out it was "very improbable" Britain would remain in the single market - to which No.10 said that was merely him "setting out his view" rather than Government policy.
But there is a broad consensus that if Britain places limits on free movement it will be hard for it to retain access to the EU's single market.
"But actually there is quite an appetite inside the United Kingdom for some kind of compromise that means, yes we will have quite a lot of access to the free market but we also have some control over immigration".
But Mr Johnson said it was possible the process of leaving could be completed quicker than some expect.
He insisted: "The two things have nothing to do with each other".
"We need to be realistic that this is a two-way relationship: that Britain can not expect to maintain all the benefits that came from European Union membership without incurring any of the costs or the obligations", he said.
Britain is widely viewed as the major stumbling block for the European Union in pressing ahead with plans for closer integration on defence and security, including transitional armies and border forces to tackle the migrant crisis. Yet the Foreign Secretary isn't alone in being slapped down by Theresa May, who has now ticked off all three of her Brexit ministers.
"I've always thought that Boris's wish was to lose by one [vote] so that he could be the heir apparent", without having to deal with "clearing up all the mess", he said.
Sterling had received a boost on Thursday afternoon after the Bank of England policymaker Kristin Forbes said she saw no need for another interest rate cut following a brighter spell of economic data.
But some lawmakers and government officials say the government has not had time to form a clear negotiating stance and that, by triggering proceedings too early, Britain may land a poor deal.