Assuming the bike is covered or garage kept, which reduce sun aging, what do you see as the useful time frame of a tire?
A quick search of the internet brings up a LOT of opinions, but not a lot of facts.
Dunlop's site seems to indicate 6 years (72 months) - at least for servicability/warrantee
WHAT IS WARRANTED
Every new Dunlop motorcycle tire that becomes unserviceable within six years or 72 months of the date of manufacture for conditions other than those which are listed under "WHAT IS NOT COVERED" will be replaced on the basis specified under "REPLACEMENT COST" below.
Metzler seems to hedge their bets a lot more:
Is there a maximum allowed age for tires?
The mere passage of time (age) does not cause tires to deteriorate. Exposure to outside forces causes tires to deteriorate. Such outside forces can include road hazards, punctures, improper repairs, misalignment, under-inflated operation, over-deflected operation, excessive heat caused by over-deflected operation, excessive exposure to ozone (such as storage near electric motors), improper storage conditions, etc. Tire companies can only have impact on a few of these exposures. Tire companies add anti-ozinants and anti-oxidants (anti-degradation compounds) to minimize degradation and carbon black is known to have excellent anti-degradant properties. Metzeler believes that since there is no way to predict what outside forces a tire will be exposed to there is no scientifically supportable age limit that can be set for tires. Some tires may be perfectly serviceable after 10 years while other tires should not be in service after only 2 years.
The easy answer seems to be many years if stored correctly and much less if not. All manufacturers sites recommend replacing tires that have any signs of ageing and/or checking (small cracks starting to appear). The all also indicate that exposure to magnetic forces, oil or gasoline, as well as UV/sunlight can severly reduce life.