How often should I have periods?

Ever since I stopped using the Deppo shot when I was about 19 I have only had a period about every 4 months. I now am 25yrs old with 2 little boys and still only get a period every 4months. I haven't used BC since I was 19 my husband just pulls out and that has worked great for us, our boys were planned. So my question is if it's healthy for my body on its own to only have a period every 4 months or so. When I mention it to my OB they seem startled and say it's not normal but they never test for anything or ask to do follow ups.