I genuinely think to redecorate 1 room a year is not unreasonable.
I have 2 reception rooms, 3 bedrooms, 2 bathrooms plus hall/stairs/landing so it generally works out each room gets decorated every 6 or 7 years.
My husband, bless him, thinks this is far too often and that if it still looks in good condition its ok to leave it. He also feels its too expensive to redecorate every few years. 
When do you redecorate ? Would you leave things more than 7 years ? Is it a waste of money ?