I for one find next to no difference at all between 8 weekly or 4 weekly work.
Granted that right by the side of a busy road or work that is close to high trees and so will make a difference but even so, that same work will be pretty grimy after just 4 weeks, let alone 8 weeks!
In fact that kind of work after just a month can often be much worse than 8 week work that is in less extreme locations!!
I guess you just need to keep learning as you go along, every house is a little different than the next and as time passes you hone your technique accordingly