You're probably right. The other companions only really got a season each, which I'd consider should be a minimum. On the other hand, I felt the Ponds slightly overstayed their welcome at almost 2 and a half season, so if Clara leaves after this season, it might be just the right amount.
Having had less than a season so far, it currently feels like she hasn't had a long enough run, but after this season is done, it will probably feel about right.
Do we take bets on how the companion gets taken out?