gram per watt theory

i've heard that growers aim to harvest 1 gram (dry) per watt of light used....can somebody with experience try and tell me how hard it is to get 600 grams every two months with one 600 watt hps light? i hope to have perpetual harvests, but is the idea to try and get 600g's over the course of two months or so? i wasn't really sure i have a feeling that's not realistic....thanks for the help.....an this isnt really a how much will i yield question, just checking to see if i understand the gram per watt theory, and how difficult it actually is...

I'm getting closer every grow! Have not yet had the pleasure to harvest a gram per watt, but i'm getting closer everytime. This grow might do it actually, maybe even more!!

To answer your question, it took me a couple grows to get the yields i was hoping for. So expect to go thru a learning curve before you hit yields like that, atleast a couple grows.