I have read different readings by Cayce on this subject. But the safest seems to be to use the Castor Oil packs for a couple of hours each day for 3 days, off 4 days.
If one is really ill then 5 days on, 3 days off has been suggested in some readings.
Here is a link (bright! wear sunglasses!): http://www.geocities.com/ecayce.geo/c-oil.htm
The link below is great....the whole site is good too!