In With Fire & Sword, the initial contract is 90 days, though renewing it will still be for 30 days. The contract can be renewed indefinitely.

Mercenary contracts last for 30 days in Warband, and can be renewed for another 30 days. There is quite a high chance that he will ask you to become a mercenary if that faction is at war, which you can then accept. To make a contract you must ask a vassal of the faction that you want to join if they have a task.

You may also be expected to recruit more troops into your party. This will allow you to fight their enemies and participate in sieges, but you will not receive any fiefs like you would if you were a vassal. To become a mercenary, you need to make a mercenary contract with one of the factions.

The function is similar, but not equal to vassalage. The player may choose to become a Mercenary for a faction.