While leaving the maintenance fee unknown to the players, it's not possible for us to know when to replace an aircraft, leaving the fee flies high...
Rule of thumb...
For planes > $300M ... replace every 6 months
For planes > $200M ... replace every 1-2 years
For planes > $150M ... replace every 2-4 years
For planes > $100M ... replace every 3-5 years
For planes > $50M ... replace every 4-6 years
For planes < $50M ... replace every 5-8 years