Not a chance in hell will the 2 years be extended. It would be in Europe's interest to see the UK fall apart and collapse, they will do us no favours. .
I have seen some crazy stuff said in here but no. No it wouldn't.
Much like how a Russian collapse would damage the EU economy so would a UK collapse, which wouldn't happen anyway.
With the loss of its financial hub (London is the only global exchange in Europe, 3rd or 2nd largest in the world depending on scale and larger then most EU exchanges combined), 2nd biggest economy, most influential member, most powerful military and strongest connection to the commonwealth, the US and the state with the most external trade connections the EU will be damaged far more then the UK.
The UK already does a massive amount of trade with nations outside the EU in deals it set up without involving the EU and as net importer from the EU individual nations would be eager to set up trade agreements as soon as possible. A 5th of German car manufacturing is sold to the UK, they will be desperate to see that that isn't interrupted. Without UK trade Germany, France and all the other beneficiaries to the EU would end up in recession from the loss of trade, they won't want to see that happen.
Would the economic hit of leaving suck? Yes. But it wont last, nature and ceo's abhor a vacuum, it will be filled quickly.