How often should you get a haircut?

I get my hair cut 2x a year. The last time I went to the salon to get them done, a simple 4" all even around, I have to go back 3x, because they can't cut straight. It took them 1 1/2 hrs. It takes me & dh about 10 mins. So, I've been doing mine, with his help to make sure they are even on the back.