You're going to have to experiment on yourself, but eccentric training causes a great deal of muscle damage and thus neccesitates greater recovery time. You can easily over do it in one session should you perform too many reps/sets with eccentric foccused training. You'll know when your body is worn out from this kind of training.
If you're trying to get stronger...you should ALWAYS do slow reps. It is MUCH more effective in building strength. Fast reps are better for training the neuromuscular connections. They are best used in the late stages of preperation for a sport.
But if you are just trying to get stronger, slow reps are the way to go. You should mix it up sometimes, but the majority of your lifts should be slow.