Say a Sahir wants to bargain with a Jinn to serve for an indefinite duration, what would the bargain modifier be? The base duration is a week, and each +3 modifier seems to multiply the duration by a factor 3 to 4 (week->month->season->year). +12 Seems low, and +15 or +18 might be more appropriate. Then again, the duration could just be extended (year->4 years ->decade->3 decades-> century...) (+12, +15, +18, +21, +24...)
And yes, this is taking into account the fact that the Sahir is breaking the rules of the Suhar.
I would say they can't bargain for more than a year. The way I read it, Sahir still need to renew their bargains with their Khadim/Khuddam spirits every year, but the bond remains throughout the sahir's life.
Letting them have longer bargains rapidly reduces a sahirs need for vis - if each pawn offered more than standard is +2 to the total, raising the difficulty by a few points to make it last a decade just raises the cost by a couple of pawns and means you don't have to bargain 9 more times. Allowing binding for a century means that most characters will summon a useful one, spend all their vis on binding it for a century, and then keep it unless they gain the ability to summon significantly better ones and haven't increased their Leadership.
From an in-character perspective, lots of magical tyoes have an upper limit to their duration, and faeries often put a "year and a day" limit to things, so why should bargains with Jinn be an exception?
It just strikes me that serving for a "year" is not necessarily enslavement or the mistreatment that would cause a supernatural uprising. Even over and over again is allowed under the current rules. My guess is that the rules presented in TC&TC show what is "generally" allowed behavior, but don't show options for more brutual treatment.
In the Exalted RPG for example, summoned Demons can be task bound for an indefinite term, or serve more generally for a year.