The correct answer depends on very many variables such as insulation, type and size of freezer, how full, ambient temperature, etc., etc. However, to increase the length of time you can be without power put a blanket or two over it as soon as you are without power and even before but do not then cover any part that dissipates the cooling circuit heat (sometimes the sides of the freezer, sometimes an element with or without fan assistance) because this will economise and extend availability of the finite power supply. Open it as rarely and for the very minimum time possible.