X3: Terran Conflict
Test this Mod out and let me know if it works. Should give you 100 times the amount of time on Assassinate missions. Be sure you're on an Assassinate and not some other type. Extract it to Mods directory and select mod from initial X3 menu.
First Kill mission I saw, I had 2 days to kill some bandit, so I think it's working.
This is likely moddable to work with all types of missions.