Read or understand? This is legalese we're talking about.
It's a lower bound. Failure to satisfy that bound can then be seen as prima facie evidence that the has not been a "meeting of the minds", due to one party being given sufficient time to even read the contract.
As I tried to imply, there are various configurable details here that would need to be negotiated; I just believe that some sort of minimum bound should be established in law. The alternative is to allow the current trend of de facto contracts of adhesion to grow.
[1] or whatever the length of time is calculated to be - I just made up this number