Settle down. Meowzer was worried because of the way you worded your post. To be honest, so was I. You don't have to let your tank "cycle" after you add your first fish.....the tank cycle we so often talk about is the first instance of bacterial buildup when you start the aquarium. By saying "I just added fish....how long do I need to cycle?" we both thought you were using the two chromis to cycle your tank..... a common, if somewhat cruel, practice that many people mistakenly utilize to start a tank up.
Now, to your immediate question:
If you cycled your tank properly during the first month (added a source of ammonia, let the ammonia, nitrite, nitrate levels do their thing...) then your bacterial filter will only need to slightly adjust to the new bioload. Essentially all you're waiting for is for the bacteria levels to increase to compensate for your new fish. I would monitor your levels of nitrite and nitrate, and make sure you don't see a spike. Give it 10 days, give or take. You should NEVER add so much at once that you see any ammonia reading, or more than just a LITTLE nitrite.
Personally I would only add one to two fish at a time, with a minimum of 10 days space between any more fish, regardless of if I see a chemistry spike or not.
Now just to be sure - how many fish are you planning on putting in there? 16g is a VERY small tank - definitely nano-status - and to be honest I don't think you're going to want more than three small fish in there..... there just isn't enough room for them to stake out their own space.
