During the last WWDC Apple revealed that there were 54 million active Mac users. If we look at the history of the product we can see that it took about 5.5 years to sell 54 million Macs. If we assume therefore that the average lifetime of a Mac is 5.5 years and knowing that Mac sales generated revenues of about $73.8 billion then we can estimate the average revenues/year/mac user: $250.
Repeating the exercise with 180 million current iOS users who purchased about 200 million iOS devices and assuming a life span of 3.5 years gives the average revenue/year/iOS user of about $150.
These are recurring figures. If we assume these users are loyal then they will likely spend this amount indefinitely and each additional user will be worth a similar amount.
So, for example, if we assume that the number of Mac users reaches 100 million then we can also assume that they will generate about $25 billion/yr in recurring revenues.
Likewise, if we extrapolate growth of iOS to 500 million users then we can assume they will generate $74 billion/yr in recurring revenues.
Adding these together gives a potential recurring income level of $95 billion/yr for installed base alone (excluding iPods, Peripherals, iTunes apps/songs, and Software sales.) Today that figure is about $40 billion/yr. 
Interesting valuation exercises can follow.  It would also be interesting to perform a similar analysis for other vendors/platforms.
- Note that actual sales are considerably higher than this figure as new customers are added. These figures should be considered “baseline” sales.
- Thanks to a kind reader for suggesting this line of analysis.