There are several areas where Lexus could do more in the name of preventative maintenance (brake lubrication for example) but that would increase the service costs and make their vehicles look uncompetitive again the competition so they don't. Dust build up within the battery cooling ducts has become a problem where Toyota now use a filter on their latest models (e.g. Lexus LBX) but on the IS300h this isn't the case.
The issue is more prevalent in hotter climates than our own but even then the issue should be apparent for months/years before it becomes an issue as the fan will just run faster and faster to compensate - the driver should hear that change and get the issue investigated. If the issue becomes so bad, the hybrid system will shut down to prevent damage to the battery from overheating.
I'd personally clean it every 6 years - it really wouldn't need doing more often than this unless you drive a lot of miles, or if you consistently have multiple passengers in the vehicle.