The sandbed and rock should be ready to support a light entry-level bioload very soon after standing in circulating water...ie maybe a couple week or so. If this LR was curing in a dealers tank, almost immediately.
Enough can't be said about directly feeding a sandbed (or your system even without fish initially) to help establish the fauna--various copepods and especially burrowing worms of various classification. These guys need protein input too, and many choose to let them scavenge bits of detritus and protein that will even be found in a brand new system (all the more if its cured LR). **There was once a time when I thought copepods needed Dt's more than anything. Might still think like this if it werent for the knowledge and friendliness of Eric Borneman through his internet forum, he pointed me in the direction if system feeding and protein maintenance...
You can provide a boost to the population density of this fauna by boosting a very important niche variable---increase food availability in the vital space (your tank). Put simply, if you will directly feed your pods and worms with crushed food of one kind or another, this will seed and establish pops faster than relying on systemic detritus.
Also, how about not putting in fish for a few months? Its only my opinion, but to make a long story short I say its better to get a hold on bacterial colonies vs. nitrogen bioload and direct feeding vs. infaunal population before introducing organisms that will tip the balance in both of these critical factors. Hopefully the outcome is less algae in the long run!!!
HTH
Brandon