Folks are throwing around various definitions of the Zone and are therefore having different (but parallel) thoughts on it.
The Zone itself is the state of automatic "point and shoot" playing, requiring no thought. The results of being in the Zone are a level of play in the top 5% of your capability.
So, of course you can't play at the top 5% of your capability all the time, by definition, because at that point it becomes your 50th% of capability, as mentioned above.
But, it is possible to play in the Zone all of the time. I have never seen nor heard of a case of it, though. The ritual goes like this: player suprisingly plays out of his mind great, he becomes determined to repeat this, he ponders how to do it again, but since the Zone is a state of a lack of consciousness, the more he tries to figure it out the more elusive it becomes to repeat the result, after a long time he gives up, later out of the blue it happens again, rinse and repeat...