I've had mine refilled once circa 105k miles when it was 11 years old. Car was left in the garage untouched for 12 months prior to that. Took it to Lexus, where it passed the pressure test prior to regassing, and never needed topping up in the two years that followed.
So I guess there is some truth to the aircon gas escaping if the system is not used for prolonged periods. Also, I read on here somewhere that the aircon gas used for hybrid Toyotas are different to ones used in normal aircon systems. As such, Toyota and Lexus dealers have a separate regassing machine for hybrids so as to not contaminate the system with ordinary refrigerant.