Hi mary, I would bag the fake rock, it will just take up valuable space in your tank. In a 90 gal tank you want to do a WC of between 5-15% depending on your bio-load per week. So 5-15 gals a week. This will remove some polutants and replace valuable trace elements in the salt.
There is not that much bacteria in the water colum, most of your bio-filtration comes from LR and LS. So the amount of water that you've removed will not start the cycle over. You actually can change as much as you want up to 40% in cases of emergency. I'm not advocating this right now but it can be done.
If you get CURED LR locally I would not think that you would not need to wait. I agree w/milomlo (
) that base rock can be a great $ savings. But you must go slow on your bio-load (adding critters) until it becomes LR. This means you're seeding the base with the LR. You can do the same thing w/LS. If you buy LR on line there is certain amount of die off (due to shipping) that you must get rid of before adding it to your tank. This not difficult and instrutions on how to do so will most likly be included with your purchase. Don't get all excited about the prices of LR on line b/c you must add shipping.
All of things we are advocating are purchases that will make your tank better and give you a better chance of success. It gets much more expensive and frustrating when your livestock dies and you have nothing to show for it.
I'm still very curious about your other water parameters, PH, Trate, Trite, ammn, SAL.
These are basics and you should have a test kit for these. Later you will want to get Calcium, Alk, Mag If you plan on keeping corals.
I would also suggest getting a refractometer for Sal/SG. Hydrometers are very inaccurate.
I'm very glad to hear you are going forward, this is very rewarding hoobby and once you get the tank going you will get sucked in like the rest of us.
:happyfish
Tim