The table shows the relative aging for a number of dielectrics.  The numbers are for reference only.  Opinions vary on the actual numbers, according to the type of test used.

          Class 2-3 ceramic capacitors go through a logarithmic loss of capacitance (ageing) after manufacture because of the slow realignment of the crystals of barium titanate after baking.  In general, the higher the K, the faster the ageing.  Because of this, electrical measurements are typically taken no less than 24 hours later (some people recommend 1000 hours).  Baking the capacitors for several hours at 130-150C restores the capacitance to its original value (should you want to, there seems little reason to do this), and the ageing starts all over again.  High voltage also tends to turn back the clock somewhat, and some test procedures take this into account.

          More information can be found at:
http://www.atceramics.com/Userfiles/dielectric_aging.pdf  

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