Pooge etc
I don't think you can give a hard freq value. Basically, the feedback divider (the two resistors that divide down the output voltage to the base of the comparator transistor) also divide down the error voltage with the same ratio. That decreases the accuracy of the loop with the same ratio. Now, for DC, that is not critical. I mean, you probably don't worry about an output voltage of 15.2 or 14.88 instead of 15. But the ac errors (noise, ripple) you really want to be zero. So when you put a cap over the top resistor, the AC feedback ratio can be made 1, that is, ALL ac errors are fully fed back to the base of the comparator transistor. But of course that only happens when the cap impedance is zero, which for a practical cap cannot be realised.
At the same time, realise that the performance of these circuits deteriorates with rising frequency. At low freq, it is probably good enough to do without the effect of the cap, but above 1 kHz or so, the cap can start making a contribution.
Taking into account these factors, I use rule of thumb to make the cap such a value that it starts to work at 1kHz, so at 1kHz Xc should be the value of the upper resistor. But there is not much stopping you taking a (much) larger cap.
Hope this helps,
Jan Didden