I agree with those that feel that it should be based on nights.
I also think that 7 night cruise would be the basis.
Silver - 7 nights +
Gold - 35 nights +
Platinum - 70 nights +
next tier - 105 nights +
and so on....
I do think they should add more tiers for people who have cruise considerably more. We were on a European cruise and they were asking how how many cruises...there was a LOT of people who had more than 20 Disney cruises.
As a disclaimer, I may be a bit biased because I know we would have reached Platinum faster if it was by nights. We prefer longer cruises, but I don't think those who like short cruises should be penalized....but I have seen some cruiser who have purposely booked short cruise to get to Gold or Platinum...so I thought DCL would somehow discourage that...but as someone else mentioned, DCL is likely getting more money with the Dream short cruises and that's why it hasn't changed.