How much and how long are child support payments due?

My ex and I have 4 children. Oldest is 20 and lives in his own apartment and has a job. Next one is 18 and lives at home. Another is 15 and also lives home. Youngest is 13. He is developmentally disabled and receives S.S.I. He bounces back and forth between living at home and living in a residency that handles kids with problems like his. He just came back home to live with his mom after 4 months in a residency for kids. Depending on his behavior, he can be home for good, or until he has another meltdown and gets admitted again. When he is home I see him at least a couple times a week and we do stuff together and I spend money on hime almost everytime we go out. I don't mind that at all. When He is in a residency, I drive out to see hime at least 2 times a week. My question is How much should I be paying for support. I have been paying 480 a month for years now. I think It should go down? I dont make the salary I used to make. Now I only make $9.50 an hour.