Well you'll all love me, not. My neighbours and i (their allotment) have got 2 weaners that will be sent off at 6months old for pork/bacon and they are on an area that is 10ftx40ft, which is about 400sqft.. not much bigger if you add the extra 8ft on to your 18ft x 12ft.
However i would agree that you cannot continualy put pigs on there, and not leave them on longer than 16 weeks (24 weeks slaughter, but buy in at 8 weeks) we aiming for one of the pigs to be weighing in at 80kg+, so yes fed 18% protein, but that said pig is a Welsh pig bred to gain weight fast. My neighbours will use the allotment for veg, up until about september time and then have more pigs on again.. to have xmas pork/bacon, then the land will be rested again till not sure when.
I wouldnt consider for a moment to try breeding pigs on that sort of sized plot. Defintely need an acre or so for that as you have all litter to deal with.