I mentioned I'd been reading Unqualified Reservations lately. One argument made there is that all governments extract the maximum loot from the population, and the difference between governments is in the horizon they have (a government with a long horizon will try to maximize growth so as to be able to steal more in future), and in the dead-weight losses involved in holding on to power.
If one considers the value of elections to be that they prevent expensive civil wars and revolutions, by making it more tempting for rival factions to wait their turn, you can get some idea of how long an elected term should be. In order to maximise the time horizon of government, giving it an interest in shearing the sheep rather than slaughtering it, it should be as long as possible, but not so long that rivals give up waiting and try to overthrow it, necessitating wasteful countermeasures.
Given those concerns, I think we could beneficially stretch the term a bit longer than five years. Even ten might be possible, but that would be pushing it. More than ten, and I think the opposition would not be willing to wait.
It might not matter. Other features might be manipulated to advantage incumbents to a degree that compensates for overly short elected terms. I can imagine that there's a sort of equilibrium - incumbents have enough power over the system that they only ever allow just enough chance of being deposed to prevent violent revolution.