...Just yesterday, I could not even crank it up though I last rode it last Saturday. Does it mean that my battery is gone?
The procedure is to remove and test the battery (step #1 is already done -- you know it's dead...). Properly charge the battery and check that it achieved the correct, full-charge, voltage. Then let it sit for hours, un-hooked, to check that it holds that voltage. Then install it and check for any current draw when just connecting the final, ground, cable to the negative terminal. And finally, check battery voltage while operating the starter (with a fully chatged battery) -- if voltage falls too low when cranking, the battery fails.
What is the average shelf life our battery if it's seldom ridden? ...should I change or should I trust the shelf life of this battery?
The shelf life of 'new style' AGM, maintenance-free, factory-fill batteries (as originally fitted in the FJR) is quite long (months).
If you don't ensure that your battery is, in fact, faulty -- replacing it with another one may just mean you'll have another dead battery later.
You need to know what caused your battery to be dis-charged (bad battery or unknown current draw?).