When should i replace the battery 2013 Lexus RX 450h

Are you speaking of the 12 volt engine battery, or the hybrid battery? The 12 volt engine battery has the same life expectancy of any 12 volt engine battery. It all depends on use and environment. Some last as short as 2 years, some as many as 8 or 9 years. If it is the original battery and it is 7 plus years old, then it is likely closer to the end of its useful life rather than the beginning. If you are speaking of the hybrid battery pack, that is something that does not have a time or mileage expectation. Drive as normal until you get a warning light indicating some kind of problem with the hybrid charging system. Some hybrid batteries last longer than 10 years depending on driving conditions and again, environment. Build a relationship with a local Lexus dealership or hybrid vehicle specialist that you can trust and will give you guidance as time goes on.