How far to push and what is a comfortable stop??

What does this mean? How hard should I push on my conditioning? And a "comfortable stop" - Pavel uses this as a recommendation for the conditioning portion of the programs in Enter the Kettlebell and has been the source of some confusion. So what is a comfortable stop and how far should I be going for my conditioning?

First - What are your goals? and What is your training age? and What is your medical and injury history?
Until you answer these questions - do not pass go - do not collect $200.
So in determining how hard to push you must first address where you are starting from and where you want to end up.
If you are a general trainee you will have a different set up from a hard core military trainee who needs to know how far his body will go before it breaks or he loses effectiveness. Individuals will fall across a broad spectrum of goals and needs - far too many for a blog to cover (well - maybe in time). So be sure you address this and then make sure your training reflects these needs.

Second - What is a comfortable stop? In strength training we can find that "failure" point and know that a comfortable stop would have been a 2-3 reps prior to that. In conditioning this can be a harder thing to define. But I take a comfortable stop to mean that you have reached a steady state of effort that you could maintain but you choose to save it for the next set or next day. If you reach a point of questioning whether you are going to complete the next KB snatch or interval around the track - you went too far. Pavel has a quote in Power to the People -"After your training you should feel ready to battle for the kingdom!" And while meant more for the strength workout (although strength training was the form of conditioning of the day that this quote comes from) I think it applies well to the general trainee.

After your training you should recover well and feel ready to do battle! If you find your training leaving you destroyed and weak and beat for hours afterward or even into the next day - it is too much. Not that it isn't useful to find your limits every now and then. And for the hard core MMA, Ultimate fighter, wrestler, military trainee it can be essential that these people find the raged edge and then recover from it.
A firefighter that is in the habit of finding his maximum level of effort may find him/herself too exhausted to perform the duties necessary to save lives - Not good.

Rotate your effort and intensity and accept that you are supposed to have easy days. Maxing out and finding your limits has its place but not frequently.