If you are doing them for meat, then as they were wormed when you got them, should be no need to redo, as they would be due about the time they go off, so I would not bother again.
If you are keeping, then I would worm every 6 months or so. It does depend on how wormy your ground is, and your view on chemical cure. Wormers only kill any worms present, they are not a preventative.
We use Panacur you get one for pigs and cattle from the agricultural merchants, which is a pellet you just add to their feed - no jabs - and works very well.