Choline Dietary Guidelines - From the National Academy of Sciences

There is an excellent (though rather old - it was published in 1998) dietary reference guide for choline available freely on the web.  If you want to learn all you can about choline, and have a basic understanding and interest in science - this is a good place to start:

Choline Dietary Reference Guide

An interesting excerpt from a related summary of the guidelines:

Maximum Recommended Safe Dose of Choline for humans:

The tolerable upper limit (TUL) is the highest continuing daily intake of a substance that is likely not to cause side effects for the target group. For choline, the TUL for adults -- including pregnant and breast-feeding women -- is 3,500 mg (3.5 grams) per day. For individuals between 9 and 13 years old, the TUL is 3,000 mg (3 grams) per day. The TUL for children between one and 8 years old is 1,000 mg (one gram) per day. No TUL has been set for infants under one year of age.

Reliable evidence suggests that lecithin and choline may protect against liver damage and may help lower cholesterol levels. Their ability to improve memory and other cognitive functions is uncertain, however. More research is needed to document possible effects of lecithin and choline on mental functioning.

Risks

No absolute prohibitions have been placed on the use of lecithin or choline.

Another Source (The Physician's Desktop Reference) suggests: