How long did Bulk & Skull stick around?
Both of them were part of the main cast for the first 6 seasons, all the way through PR In Space
. Bulk stuck around as part of the main cast for one more season, PR Lost Galaxy
(though only in 4 episodes), with Skull only appearing briefly in the first episode as a guest star.
They both returned to the show for a cameo in the 10th Anniversary episode "Forever Red"
Bulk returned as a member of the main cast in 2011 in PR Samurai
, and its second year PR Super Samurai
. Skull made an appearance in the last episode of Super Samurai.
(Because they got jobs in the UN! Pfft, were all the departures like that?).
As far as mid-show departures go, no, none that I can recall were as dumb and contrived as that World Peace Conference nonsense.