There wouldn't be a rule of thumb, but you could setup a test.
Growing up we had four goats clearing underbrush and lower branches on trees. Pigs will likely work faster. After you define your paddock and do your initial walk through to make sure there isn't anything that is poisonous, these goats would clear a quarter acre in a couple of weeks.
Clear means they removed all the small branches, bushes, and undergrowth; you could walk through the area easily. Unwatched they would eat everything six feet up the tree. Of course, for you this would depend on the type of woods you have, how many pigs, their level of activity, but you could test a quarter acre paddock.
Journal the time as you move them around, then extrapolate for the total acreage you'd like cleared. Meanwhile, your early paddocks can be put into use without pigs revisiting or waiting till pigs are completely done with entire property. Look for Joel Salatin's Pigaerator videos on YouTube, might give you some ideas.
Who knows, you might end up with some lazy pigs, but after that first quarter acre you can get in there to plant something else.