1.7 to 9 days good enough to spend time in Andaman.
2.Try to spend maximum time in Havelock where apart from its beautiful scenic beauty & beaches you can also enjoy different types of water activities like Scuba Diving,Snorkeling,Fishing etc.
3.While in PB definitely visit Jolly Buoy for snorkeling.Other places Cellular Jail, Corbyn's Cove Beach,Naval Museum(Samudrika),Ross Island,North Bay(if interested in Under Water Sea Walking).
4.If you get time you can visit Neil Island also.
5.Since you are travelling in Jan which is a peak season there do book your accomodation well in advance.If you want to travel in Makruzz (the luxury vessel) to Havelock you should book you ticket early.You can book online also from their web site.