I tend to stay in most roles until I have all the level 7 magic that the role has, about 11 or 12 levels. Barbarian is different, the longer you are in it the better your hitpoints are. My solo whiska wizard has been a barbarian for 18 levels now but I am thinking of changing to ranger just to obtain more gold.
The undisputed legend of the Gael Serrin is Felix the assassin, he was a barbarian for 22 levels.
It depends alot on the type of team you have, it is probably worth leaving one as a barbarian for a long time as long as other charactors can get the skills that you miss out on by not changing roles.
Also remember that you should be a barbarian when you find the idol of bongu (or was that bungole or maybe bungleau) so that you can get the giantstrike trait.
Like most things in this game it depends on your priorities in your charactor development.
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