It really depends on how long the battery is disconnected. There is residual magnetism in the pole shoes of the generator, but if disassembled or if the generator sits a long time, this magnetism dissipates. That's when it becomes neccessary to re-polarize. It sets the proper north-south relationship on the pole shoes and re-magnetizes them. How long this period of time is I couldn't tell you. I've seen poles shoes lose their magnetism within days, but most of the time it seems to last many months and sometimes longer.
Those "one-wire" alternators that seem to be popular replacements rely on the residual magnetism in the rotor poles to get them initially charging. If they sit for a long period of time and lose that magnetism, they'll never get going unless they get "juiced" again.
Edit: One more thing, just as a point of interest. When the armature is left in proximity of the pole shoes, the magnetism retains for a much longer period of time. Once the armature is taken away, the magnetism seems to dissipate much faster. We've run into this problem at work with late permanent magnet type starters. If the housings with the magnets in them are stored too long without armatures in them, they will start to lose their magnetism. We process the stuff too quickly through our plant to worry about it, but once we bought a bunch of new permanent magnet field housings that turned out to be no good because they were improperly stored. We had to send them back to be re-magnetized.