It's difficult to quantify without knowing the available adsoption of the zeolite (which would probably be expressed in moles of NH3) and the NH3 production of your tank.
If there's some adsorptive capability remaining, it should grab the NH3, so I believe that it would have a pretty steep drop in activity. Nothing is really on/off, there's always some rate of change.
I believe that the drop in activity would be pretty fast because the adsoptive capability of the amount of zeolite added to a filter is likely to be a great deal higher than the amount of NH3 produced in a day. In other words, initially, everything would be snatched up, and fractions of the zeolite would be filled each day. Eventually there would be a fraction remaining that would be close to the amount of NH3 being produced, so one day it may take it all up, the next there may be a little to show, but the third you'd have the full amount being produced free.
Think of it like filling a cooler with brownies. Every day, someone gives you a brownie and you have to put it in the cooler or eat it. Let's make them poison brownies
. At first, you have no trouble fitting them in, but after a while, there's room for one and a half. The next day, you break a brownie in half, but you have to eat the remaining half.
Yes, nitrifiers will pick up when this final stage is reached, but I think that the zeolite will fail at a much faster rate than the bacteria can grow to match. Plus the ammonia nitrifiers are limited by the zeolite adsorption in much the same way as NO2 nitrifiers are normally limited by NH3 nitrifiers, so there's another level of restriction placed on nitrifier growth, which I think will be more detrimental to NO2 levels in the tank that NH3, since NH3 nitrifiers can grow more quickly.
But, as I said at the beginning, without some kinetics experiments, this is all just an educated guess.