I believe each store has different business needs and of course, different availabilities from their CM's. I know the MINIMUM availability is 3 shifts a week, some stores may require more. Also, at least ONE of those shifts must be a weekend. I have a 40 hr. week full time job, so i have full evening availability and full morning/night availability on weekends. It is definitely worth my 1-2 shifts i get to be there. During the holidays, this can increase to 3-4 shifts a week and very late hours. After 3 years, your discount jumps to 35%, stays 20% on your meals. You get little extras like free lockers and strollers, backstage access at the parks to buy exclusive CM merchandise, this includes the lost and found areas.
I have added on to my DVC 3 times with my 15% discount on points. When they gave us free park admission, we all thought it was too good to be true, but it's been over a year now and it is still happening! It's really fun to help others spend their money and help plan their trips to the world. Good Luck if you decide to go for it.