that all depends on your taste and what you would like. In my tank, i think i have alot of rock, although i plan on adding some more on my left side of the tank (where my overflow is)
I do plan on making it into a reef tank, and made sure i allowed some space or easily moved rocks in case any corals i get come with big pieces of rock. But other than that, i wanted to create a wall, and that took lots and lots of rock. alot more than i thought.
about 235 lbs in a 135 gallon, along with almost 400lbs of sand (only 60 of it is live)
But as a warning... its been over a month, and my tank is still cycling.
probably because i ended up getting some uncured rock (which was a bit cheeper)
Some helpfull advice.. if you plan on making a wall, try and get some large peices of rock instead of smaller peices. They stack much easier. I also looked for some nice flat peices which i created "shelves" for stuff to grow on.
Good luck