Usually the longer you wait the better for achieving an stable environment. Although there is always too much emphasis given to "complete the cycle" once Ammonia and Nitrites are undetectable but IMO there is a lot more to establishing a new system than just the bacterial cycle including your familiarity to it and to establish your husbandry maintenance routine.
Many new starters start adding fish when there is no Ammonia or Nitrite detection but loose a great opportunity to get rid of the initial Nitrates and Phosphates so, soon after; they get cyano and hair algae and with start calcium and alkalinity issues.
Get familiar with the chemistry aspects of your tank, try achieveing Calcium, Alkalinity and Magnesium balance also. Algae will be easier to control without corals or fish because your need less lighting and feeding is minimal or none so you can have whatever nutrients completely out, it will allow to lower your Nitrates and Phosphates also to undetectable levels letting whatever phosphate came with the rock leach out. It will also let your pod population reproduce before introducing predators.
I would complete adding whatever clean up crew is still short off and let the system complete the algae cycles, keep on with water changes (10%/week min) and watch on Nitrate accumulation.
Track your Calcium and Alkalinity levels and get familiar with a supplementation method that works in your system. You do not want to experiment when fish and corals are present so the best time to learn is now.
I have known people who wait up to six months but IMO one month or a month and a half is enough to let any predatory hickhickers to show up and be taken out as well as most potential fish parasites to hatch and die without fish. Also will help you achieve calcium, magnesium and alkalinity stable and on target levels (420 ppm, 1300 ppm and 3 to 4 dkH) and will help you become familiar with how your system react to your supplementation method.
So you can definitely buy it now and your tank should have good stability by the time you are ready to put the fish in.
