There is an Autotrans bus from Mali Losinj to Zagreb, but if you can break up the trip (somewhere like Opatija) then I suggest doing so. There is also a Krilo catamaran to Rijeka, but it may be seasonal--check the dates,
I would like a week on each, but as I said in your other post, it just depends on what you want out of the trip.
Would we need a car to get about and if so can we hire one on cres or losinj?
We did it without a car, so it can be done, but you would miss out on some things. We were just relaxing and hiking, so it was fine, but when we return it will be with a car.
Contact the tourist agencies to ask about rental: http://www.tzg-cres.hr/en/
We rented our apartment from this agency and they were extremely helpful: http://www.island-losinj.com/