Green severums in the hobby are basically a long standing, captive bred, genetically mixed bag, so they can vary in color, be heavily spotted, or have few to no spots. Some call them efasciatus or even severus. Heros severus is a geographically specific fish with specific characteristics, so common green severums are certainly not severus (though it wouldn't surprise me to see severus hybrids). What's currently known as efasciatus are widely distributed in the wild and can vary in different locations, but to me calling hobby severums 'efasciatus' is somewhat arbitrary. I suppose you have to call them something, but, for example, while some look like efasciatus, I've seen some resemble notatus. The so called 'red spotted green' severums I've seen basically look like nicely spotted greens to me, but who knows what their lineage is.
Anyway, that aside, I bred rotkeils, raised many, and most of mine easily got to 6 inches or more within a year from fry. It's been over 20 years since I had common green severums, but I'd expect 2-4 inches to take less than 6 months, more like 4 months on average.