My Macbook Air, Mid-2012, has said "Service Battery" for about two years now. The battery would die after about an hour of use. If I had any accessories plugged in (DisplayPort -> HDMI converter, for example) it was game over in 20 minutes.
Super easy repair. Everything comes out with screws and the battery cable is removed with a small pull-tab.
This is way easier than any iPhone repairs.
Don't wait. If you're thinking about it, just do it.