One lateral move at the regional level shouldn't be a problem, there are all kinds of good reasons for that: Geography, better compensation, faster upgrade, etc. Any explanation is good except being "asked to resign". Most pilot interviewers know who the real doo-doo regionals are, and will respect you for not taking their crap any longer than necessary.

In the business world, the ideal tenure is 5-7 years. Much less and they will think you jump ship too often...any longer and they suspect that that you are lazy, complacent, and tend to stagnate, unless you have continously advanced up the ladder.