PPM strength is mostly strain specific. In my experience, indica plants tend to like a higher ppm than sativas do. I will illustrate the methods I employ.
Vegitative stage:
When you are using seedlings and rooted clones, the ppm will need to be low, in the 300-500ppm range is usually sufficiant.
note:think babies, the tiny plants don't have enough solar panel to use high nutrient levels yet.
After the leaf surface area gets larger, the plants can uptake more nutrients.
When the plants reach about 2 weeks of age, gradually increase your ppm
to 500-800 ppm.
If you get a little leaf-tip burn, just use a slightly weaker solution.
As the weeks progress, gradually increase the ppm to a maximum of 1200ppm
in the vegitative stage. My plants will usually be 7-8 weeks old at this time.
Flowering stage:
So the plants just got about 7 weeks worth of nitrogen rich nutrients but will soon be switching to more of a phosphorus/potasium based nutrient.
In hydro/aero; I start my flowering schedule with a moderate amount of nutrients, in the 900-1100 range (depending on the size of the stalks and whether indica or sativa.)
Then as you progress through the weeks, higher nutrient levels can be used by the plant easier as the metabolism has now fully shifted into flowering mode.
I generally take indicas to about 1500 ppm max during the 6th week, considering we're on an 8 week schedule.
The last 2 weeks I use only water and some flushing solution for the first few days of my flush.
All the while maintaining a 5.5 to 6.2 ph.
The last 2 days I use water and Good old black-strap molasses. The flavor is absolutley amazing when the buds are dried nice and slowly.
I hope this helped you.