You actually do not need much actual LR
, 20 lbs should do it. In a 90 gal
tank, you may wish to have a bit more rock in total though.
The time frame will vary but in very little time the bacteria will colonize the surfaces and the fauna will migrate in a few short months. As far as the surface area's for coralline and such, that will greatly depend on water chemistry and lighting but it could take anywhere from a few month to a year.
One alternative to LR
completely is getting some Grunge
. It will have many types of coralline as well as beneficial "critters". Basically all the good and none of the bad...