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PTC thermistor question

Bdale Garbee wrote about PTC thermistors as crystal ovens:

Just make sure you let the unit run with the PTC on for a few days before you get too frantic about trying to measure or adjust the frequency.  Once the crystal settles in at the new operating temperature, it'll stay there.

Wayne wants to know:

Why do you recommend running the crystal oven for several DAYS?  Wouldn't the crystal temperature stabilize in an hour or two?  Wouldn't the crystal frequency stabilize once the temperature stabilizes?

Is there a problem with thermal shock if you don't keep the PTC thermistor on all the time?  What I mean is, will the crystal fail prematurely if it is occasionally heated rapidly from -20C to +60C?

On a similar topic, can anybody report the "real world" temperature stability of the LO in the DEM and SSB 2400 MHz down converters?

Wayne Estes W9AE
Mundelein, IL, USA

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