Hire car after a non-fault accident

Hi all
We had the back of our car taken out by a Tesco lorry.
Their insurance company arranged to pick up our car and dropped off a hire car the next day. Good service.
Our car has been found to be too damaged to repair so we have had an offer which we have accepted.
My question is how long can we use the hire car for after we get our payment, their company have said that
we can use it for a week to let the cheque clear but a friend of mine has said that we are entitled to it for up to three weeks
to give us time to find and go and get the new car.

So is my friend right, if so how would i go about negotiating this with their company.

Thanks