Principles of training
A number of people have asked questions about how much training is enough, and how much is to much. Both of these are directly related to the Principle of Training concerning systematic and progressive overload.
There is no easy answer that will tell you what is "enough" training and what is "too much". However with the help of a calendar and knowing when your main triathlon goals take place in the season, you can make an educated guess.
The main goal of training is to adapt to the effort. Eventually you will adapt and the training will not be a challenge any longer. As such, you need to gradually increase the training load to keep your body adapting. Simply put the training load can be either added volume (more hours at the same effort level), added intensity (more effort over the same duration), or a combination of the two.
"Too much" training can be either too much volume, too much intensity or too much intensity and volume. Regardless of the source, all sources of "too much" contribute to fatigue. Unmangeable fatigue results in less speed, less endurance, less strength, less motivation, more injuries, etc. Your program stops yielding results.
"Enough" training is any training load sufficient to get a training adaptation. This load needs room to grow continuously through your training program. When growth ceases, adaptation will also cease. Your program stops yielding results.
To figure out the balance between "enough" and "too much", a good coach will work back from your main goals in the year, and factor in feedback from your previous season. With a little planning you should be able to figure out a program that slowly and progressively overloads your abilities requiring you to adapt at a rate you can handle.
A really well designed program will prescribe a progressive overload that continue from one season to the next so you don't always start at square one again after an extensive period of recovery and the resulting detraining.