League of Legends (LoL) Question: How long does knights vow last?

Knight's vow last for as you don't change your partner or any other of your team members don't make him his partner (in this case it will change for the last one).

Two thing you need to keep in mind,
1) You can change your partner every 90 second (if you don't have Ruby Sightstone), it useful sometimes especialy late game middle team fights if your first partner died.
2) The passive of Knight's vow work only when you in range with your partner.