How many days of Notice period we have to serve when unallocated?

It totally depends on HR and RMG, if they don't want to spend money on you without any actual work, they can release you in a single day, but if they want you to struggle then full 90days 😁

If you are unallocated and don't get a project within 10 days then RMG may early release you. You can ask them about it.

90 days it has nothing to do with your allocation