How much work experience should be included in Statement of Purpose for Master program?
Well It really doesn't matter much whether your research subjects are related to what you have done internships in!

To make it more clear. Internships depicts that you are inclined and willing to work along with your regular curriculum. I would suggest you to relate your  internship with research subjects. As we all know that all subjects in Computer Science are inter- related so try to find out any open source concept that you can relate to or willing to work for it. This much is enough. No doubt Research is most important aspect for admissions but you should know to combine it with your life. Give small Instances (backed with Proofs) where you can prove your point. Speaking about Number of years to show for experience is all depends on your profile, show max possible and make sure that you have salary slips.