How often do you apply more color?

Hi everyone! Ive been using Punky Colours for about 6 months. I have all natural mousy light brown hair with natural blonde highlights. My hair is fine and sun bleaches easily when Im outside in the summer, so that where the lighter pieces come from.  I have Punky Plum right now. I like it a lot. I notice that my roots fade away first, after about a week. The rest of the hair does lighten up, but after 2 weeks still a pretty good about of color except on the roots. Im wondering if I should apply just on the roots now after 2 weeks, and do all of it in 4 weeks, or if I should just cover all of it again to make sure that it matches better. How often do you guys reapply your color? Would it be crazy to go over all of it every 2 weeks with more plum? I tried adding some color to my conditioner and doing that when I shower, but it really didn't stick on the roots. When I normally do the color I do it straight from the bottle- not diluted and it works great. 🙂 Just not sure if I'll mess up the color if I apply more plum over plum every 2 weeks, or if it will just get it back to where I wanna be- all plum? Thank you everyone!  😀
This picture is about 2 days after I did fresh punky plum over mostly faded out plum, over my natural.