The oil based forms are not well absorbed, so at least 300mg a day. This type is commonly in stores.

There is a new water soluble type, Q-gel, etc, which is better absorbed. 100mg a day of that is probably enough.
example:
http://www.epic4health.com/noname.html

Costco just started carrying Qunol, ubiquinol, at quite a savings.
I am trialing it now, to see how it works. I am so far 30 caps into a 120 ct bottle, once a day. 100mg each.

You have to read labels to see what you are really getting.
Puritan's buy 1 get 2 free is still going. See what they have there too.