Here's the deal:
You get mummy rot and every minute there is a chance that you will loose a point in charisma and constitution if you fail a saving throw.
However, every 30 seconds damage to con and chr is reduced by one.
For this reason mummy rot is really a joke despite the fact that it is permanent.
With the per 30 second reduction in ability damage, this kind of effect has really almost no meaning and influence. This type of damage should be given a boost to last for perhaps 5 minutes. Then restoration potions would also be worth one of your inventory slots.
(I tested this in the Sands..)