multiple entry visa for 2 years.

I'm trying to apply for a multiple entry for 2 years. I've been going to china to visit my family, and friends very very often, almost 2-3 times a year and its been a huge hassle to go get a new visa every year. What requirements do they need for a 2 year multiple entry with 90 day each stay? My parents and I (26 year old) are all Chinese born naturalized american citizens. I have most of my extended family in china, grandparents, uncles aunts, cousins etc... We own property in china as well, so i usually stay at my own place.

Also, since i usually visit multiple families and friends during a stay, my stays vary fro 30-90 days. What should the person who is inviting me say on the invitation letter? Thank you