Dear Sreenath
For public limited companies, offer shall be opened for not less than fifteen days and there is exemption to private limited companies in case ninety per cent. of the members of a private company have given their consent in writing or in electronic mode, the periods lesser than those specified in the said provisions shall apply.
Hencefore, if your company has given the offer letter limiting time not less 15 days in the notice, it cann't make the allotment before than fifteen days, even though it receives all acceptance letter / decline letter from shareholders, it has to wait for 15 days for making allotment.