No other criterion is more important than this: there MUST be mutual agreement for friendly matches!
Reasons:
1. your friends can be much stronger than you, and your team will be the one over exhausted and risking to injuries
2. your friends may live in different time zones and you can do nothing about your surprises while you are sleeping
3. Sometimes you need more than 3 hours for resting your team. Not every players recover so fast!
4. Your life is not just about playing matches, managing also involves training and rest, and to know and decide when each of your players can play a full match.