Short Answer:  150 square feet
HD-80 should be applied heavily at a rate of about 150 square feet per gallon.  The stripper is designed to soften existing stain and sealers to a near liquid consistency.  The amount of time this process takes depends on many factors.  The type/brand of finish being removed, the number of "coats" or applications, the amount of direct sunlight the wood receives and the amount of time since the last application all play a major role.
HD-80 stripper is only active when it is wet.  If the stripper begins to dry before the existing finish has softened enough, more stripper should be sprayed on to keep it wet.  This will reduce your effective spread rate.
A high quality oil based exterior stain should be reasonably easy to strip.  The "dwell time" or the amount of time the stripper needs to soften it thoroughly is usually about 15 min or less when the temp is about 70 degrees.  Under these conditions you can count on that 150 square foot per gallon estimate.
If you are stripping several layers of a cheap water based, or worse a water reducible alkyd based product the dwell time is going to be much longer.  Film forming products that crack and peel are often the most difficult to remove.  Sometimes you have to keep the surface wet for up to an hour to completely soften all the layers to the point they can be gently power washed off.
If you are doing this work in the direct sun, on a breezy day you are going to have to apply the HD-80 several times in order to keep it wet that entire time.  Each time you soak the surface your spread rate is going to be roughly 15o square feet per gallon, however your effective rate for the project might be half that or less.
This is the main reason professional contractors will almost always perform a scratch test to determine how long its going to take to strip the surface and better estimate how much stripper they will need to complete the job.  For the DIY community we offer sample test kits so you can determine how easy your wood will be to restore.