I'd say it'd take regular snacking on small amounts of carbs every couple of hours to achieve hanging around in almost-ketosis for days on end.
I spend a few hours each day in ketosis and it's an easy transition from not being in ketosis. Often I work out while in ketosis.
There's nothing magical or specific to PB about ketosis. Most people on a SAD go into ketosis every night during the last couple of hours or so before awakening, when their liver runs out of glycogen.
F 5 ft 3. HW: 196 lbs. Primal SW (May 2011): 182 lbs (42% BF)... W June '12: 160 lbs (29% BF) (UK size 12, US size 8). GW: ~24% BF - have ditched the scales til I fit into a pair of UK size 10 bootcut jeans. Currently aligning towards 'The Perfect Health Diet' having swapped some fat for potatoes.