You could replace the restore image every month, but why?
that would potentially indroduce an unstable build to restore to in the event of a crash. If your data is what you are worried about, I suggest following my process for a 4 partition system, I use it with no worries.
I redirect the MyDocuments folder to the D drive (Data) and do a restore every 3 months.
I am also currently on my 4th version of my image, but always make the image diectly after installing...