Buy a test kit and check your ammonia, nitrite, and nitrate levels. If the live rock is cured it could have the bacteria needed to speed up your cycle. If not, it could have the die off needed as an ammonia source to begin the cycle. I wouldn't add a fish until you're showing 0 ammonia and 0 nitrites. My cycle on a 75 took about 8 weeks and it was rough! Lol I added live rock which gave me something to look at and find the little hitchhikers that come with it. I say add the live rock and not the fish until your water test shows it's okay. I don't know of anyway to look at a tank or a specific time frame in which to say a tank is fish safe.
Owners are passionate about the hobby and understand that your success in the hobby equals continued success for their business in return customers. In contrast, my experience is many LFS
employees just want to sell you something whether safe or not and many times you are have much more knowledge than they do. I've learned, an IPhone is a great thing in a LFS
. You can search the net quickly and review the forums for the proper information before buying.