The manufacturer recommends 1 1/2 oz per qt, however I would question doing it in a motor with no fine particle oil filter.
Seafoam is a detergent and will clean deposits from the inside of the engine and put them in solution in the oil. Do you want all that gunk being pumped to the bearing and wear surfaces?
If for some reason you do feel this is a good idea I would add it before an oil change, take a 15 or 20 minute ride and then change the oil.
Most people who are using Seafoam are using it as a fuel system cleaner, at which it excels.