To cover complete sightseeing of Arunachal Pradesh , you will need at least 30 days.
The Tawang region, with sightseeig of Bum La Pass and Zimithang, can be covered in 9 days. The Ziro and Mechuka region will take additional 8 days to cover (if you continue from Bhlukpong, after covering Tawang). The Anini region, through Mayodia Pass, will take additional 5 or 6 days to cover, if you continue from Pasighat (after covering Mechuka). If you wish to continue to Dong area through Tezu, it will take additional 5 days, from Pasighat. To cover the Namdapha and Tengapani area, it will take additonal 4 days, from Dibrugarh.
Note that these routes only include regular sightseeing. If you're intrested in special activities like birding in Eaglenest Wildlife Sanctuary in Bomdila area, or exploring spiritual Buddhism in Tawang area, or interacting with tribal people from different parts, it will take additional time.