Anytime. FWIW, some angelfish have a habit of nipping at corals. If you have the budget and depending on the size and quantity of fish you want to have, I recommend having separate QT tanks if you're going with multiple tangs or angels.
Independently, if you add fish at different times and let a few fish go through the process before adding new ones, one QT tank will be fine. This is helpful if you wont have fish that areaggressive towards the same species.
If you plan to have small less aggressive fish, one 20 gallon tank, preferably long will be sufficient. A 5ft tank with a divider is doable but not suggested as if one fish have ich or any other diseases, the fish on the other side of the divide could also become ill and so forth.
I had dirty fish so I performed 25-50% WC's while fish were in QT while treating for copper and other diseases in case.
Leave the tanks running for as long as you continue to add fish or feel you may impulse buy others as you go. I left my QT tanks running until I finished adding all my fish.
Shortly after I took them down, and now it takes away the urge and temptation to buy new fish a year later