When does the Charcoal Vapor Canister have to be replaced on a Lexus ES350

Unfortunately there's no way out and it will not pass smog with check engine light on and you can not reset the light by disconnecting battery cause it will show that is hasn't completed scanning all sensors

If the EVAP system is causing the CEL then you probably have no choice but I would shop around for a better price. That sounds like dealer gouging.

I would go to another shop where the rates are not as high as the dealership. Looks like there a two different styles of Vapor Canisters. One is for $65.00 the other one is $520.00. So I guess you have the more expensive Vapor Canister. The reason you are in the situation of having to replace this is you must be topping off your gas tank when filling it up. This overflows into the Vapor Canister and ruins it. You need to stop pumping gas when the nozzle clicks off the first time. Unfortunately you need to have it fixed to pass emissions.

If money is tight a used canister might fix the problem. Of course if it does not work you will be out the labor costs.

I have a 2010 lexus 350 with 186k km and my check engine light came on. The diagnostic at the dealer was the same problem and to replace the canister would cost $1200 parts and between 2 to 4 hours labor since the access to the canister is very difficult. They removed the check engine light and it hasn't come back the last 4 days. Should I wait to replace the canister when and if the check engine light appears again?

I would, if you don't have an emissions test coming up. Not going to hurt anything at the moment.

Will it help with gas mileage