I wouldn't quite say never lease planes out for more than a year. It really depends what price you're leasing them at - for me, I try to keep it around 10%. I often lease planes for 3 years, which I would say is a fairly reasonable amount. By then, maintenance costs are approaching the value of the lease. People who lease for 10 years or so are going to be making losses by the end of it unless they are charging very high rates (which, might I add, is not inconceivable - leases made early in the game can be very high, and if the airline goes inactive...). One must consider that some airlines out to lease an aircraft will pay a premium to get a longer lease - as it means they need to change routes over less, and have a guaranteed aircraft for longer.
The problem with leasing large aircraft is not so much that maintenance is huge, but that lease revenues are small. If you charge 10% on a Fokker-100, that's 3.7 million / month - quite affordable. If you charge 10% on a 747-400, that's 38 million / month - the price of buying outright the Fokker which will earn just as much doing domestic routes.
With larger aircraft (as in, around 100 million plus), it's quite difficult to make money by yourself. When you factor in the owner taking a cut, it becomes even more expensive. This is not to mention that the expense is more visible - as you can clearly see how much it costs per month in lease fees, and how much (less) you're getting in fare revenue.
