Major upgrades usually occur every two years or so. There's no set arrangement for the frequency of upgrades.
Minor updates occur throughout the life-cycle of the current version. These are, of course, free.
When you purchase a license through the Moneydance website you are entitled to the next major upgrade for free. It is my understanding that you will NOT receive the same offer when purchasing through the Apple Store. The Apple Store version is also slightly crippled due to Apple's sandboxing requirements.
Upgrades are not forced. You can use the current version for as long as you like. I continue to use MD 2011 for example.