Doubt it will be Shorter.
You have no Idea just how Extensive the Contracts are.
And you also Forget that neither Side can give in.
If UK Accepts the Regulations etc from EU which the EU WILL demand for Market Access. The UK would be Humiliated. And given that the Referendum alone is already Tearing UK Apart with Independence Movements Gaining Momentum all over the Country. This is just not something the UK would Survive.
The EU is the same. If they were to give UK Free Access to the Markets without the normal Regulations like Norway and Switzerland. The EU would be Humiliated. And all the Countries in which there is Anti EU Movements would Gain momentum to Leave as they get a good Deal outside EU without the Rules and Regulations.
With this Preset the Negotiations will likely Drag on very Long.
They could end up taking 10 Years or more.
Or even Worse the Negotiations could Break down and UK will be Forced out of the EU after 2 Years without a Deal at all.
You are absolutely right, and there is a clause that, if all parties agree, the negotiations can be extended .. we will have to see how it pans out.
It is an uncertain future that's for sure ..