I'm not sure if there are "rules of thumb" but I can tell you what I have come up with.
If you look at the chart of Raj Apte's site (near the bottom), it shows in a mixed culture the time range of the dominant microbe.
For me, I taste at 6 months, 9 month and a year. At the 6 month date, if the sourness is pretty low I will add .5 - 1 lb maltodextrin depending on my feeling. Then at the 9 month point, I will see how the beer is progressing and let it go or add fruit. The amount depends on the type of fruit and the flavor or extra acidity I want. I don't add any flavors until I have tasted the beer at the 6 or 9 month mark because until then I don't know what direction the bugs have taken the beer (I have a guess while designing the recipe). I try to match my fruit or other flavoring component to compliment a flavor I'm tasting in that beer. And sometimes the beer doesn't need anything.
This is just my method I have developed but I am still in the early learning stages.