Asssassination Contracts currently last for roughly 30 IRL days.
However, there have been instances where you place down the money for that contract and people start getting very evasive and don't play the character to avoid the contract.
Which is why I propose the following:
Assassination Contracts Expire after 30 IRL days if they player has reached 120 hours of playtime on that character. If they do not play for 120 hours of playtime in that 30 days, the contract expires when they reach 120 hours of playtime total.
To clarify here. I'm not saying the playtime counter 'clears' every 30 days, but they just need to play the character 120 hours total before the contract clears so the assassins atleast got a good shot at catching the character online.
I am also not saying it should last shorter than 30 IRL days.