Change the oil, and put 1/3 of the can in with the fresh oil. It does not change the interval.
Have someone start the car and rev the engine slightly so it won't have a hard time running (2000rpm is fine). Pull off a larger vacuum line (brake booster works fine). You want to pour 1/3 of the can into the line. If you suck it in, don't drop the hose in the can. The engine will stall.
As soon as you have 1/3 of the can done, shut the engine off and let it sit 15min. When you crank it, it will billow enough smoke to cover a couple city blocks.
Once the smoke starts dieing down some (normally about 10 min of idling) take the car for a drive until it stops smoking (at least for the most part). During that drive, you will want to hold it in a low gear and pull the engine through it's rev limit a few times to help burn the carbon out. Don't drive like that all the time, but be sure to do it.
You'll want to repeate the intake a second time with the remaining 1/3 of a can.
Buy a seperate can for the gas. Drive until the tank is near empty ( FYI It's an 15 gallon tank.), pour the can in, and fill with gas. It may smoke very lightly on that tank of gas... You *are* burning seafoam with the gas remember!
Seafoam does a great job at cleaning the fuel system, the oil in the crankcase, and some parts of the intake track, like the heads, and valves, and as good a job as anything else with cleaning carbon out of the combustion chamber (Nothing but water does that well).
It is still not a replacement for taking the upper intake off and cleaning all of that by hand.