How long to allow for mortgage?

Hi all

I am buying a property with a mortgage. The seller is mortgage free, and is pushing hard to close the deal as soon as possible.

Form F allows 30 days as standard to complete the transaction, after which the failing party owes 10% to the other.

This seems too short to me for a bank to process a mortgage, so I want to add a clause allowing for a grace period in case of delays by the bank.

How long is appropriate to allow for that grace period ?

I have had friends suggest 30 days grace is normal - is that correct ?