Be careful going down the 'fair route' i.e. a long PoW phase unless you have a seriously good retargeting system. The native Peercoin and Novacoin retargeting on every block doesn't work anywhere near as well as KGW, DGW, Digishield etc.

A short PoW stage may help to get and keep miners interested long enough to sustain high hash rates and add some resistance to attackers using pool-hopping tactics. These attackers love to drive the difficulty through the roof, stop mining suddenly, leaving the blockchain stuck. It happens a lot with new coins and it causes a lot of problem when legitimate people struggle to find blocks; they will just give up very quickly.

Also, a lot of miners seem to like to mine a coin for a while and move on to something else fairly soon.