Considering it's come up a couple times of late, if there isn't a sticky already, this is a question that is perhaps worthy of some discussion.
What is a cycle exactly?
I don't think there is a definitive answer to that question.
I can "cycle" a tank in 24 hrs or less if need be. That doesn't mean that it's the best way to go. I can also cycle a tank w/ a fish or two without, contrary to popular belief, harming the fish.
It all depends.
If you add enough established live rock to a tank at the outset, you may well never see the commonly accepted measurements that indicate a cycled tank. I set up my current tank over a year ago. 20g tank, 25lbs of established (i.e. very well cured) live rock, no other critters for the first month or so. Never saw a measurable Ammonia or Nitrite level. Added CUC about 5 weeks in. Big CUC. Never saw a measurable NH4 or NO2 level.
Was the tank cycled? Yes, but not in the traditional sense. Nor did it need to be.
I could put together a brand new 55 gallon tank. No rocks, no sand, nothing but glass, water and requisite mechanical equipment - heater, brand new filters, etc., (but no bacteria source.) Add a little bitty goby of some sort. Give it 6 months, and the tank will be cycled. Would I recommend that? Absolutely not. The tank will be "cycled" so to speak however, and the goby will suffer no ill effects in the process. Alternatively, add 7-8 medium sized Damsels and that's animal cruelty IMO, because they will suffer ill effects.
What "cycled" means to me, is that the Nitrogen processing bacteria are able to keep up with the nitrogen producing stuff in the tank. That stuff can be alive or dead, doesn't matter. Doesn't even have to be saltwater. One could cycle a tank with a chunk of raw beef, shrimp, rabbit, even cat poo. It doesn't matter. (Well, the cat poo may not be a good idea for other reasons, but it will work to establish the cycle.)
All that matters is that new bioload is added at a slow enough rate to allow Nitrogen processing bacteria to do their jobs.
That applies equally to a new tank or an established tank. Toxic levels of Ammonia ---> Nitrite can easily be generated simply by overfeeding.
Bottom line is, regardless of the age of the tank, the bioload should not (soft vs. hard cycle, which is, perhaps, another discussion) IMO, be allowed to exceed the bacteria's ability to compensate for it.