If you need to know the FIT data of BME280, please contact your local FAE from BST. Those data are protected by NDA. You need to sign it first.
If you are asking how long BME280 will be on the market, At least we don't have plan to phase out this sensor now which means you should able to use it until end of 2021. Even we send out formal announce EOL of sensor, we will give enough time for customer to switching to our new solution.
For the warranty of sensor, as consume product, our life cycle is normally set as 1 year. So within the life cycle time, if our sensor have any issue, we are able to help customer to debug and found out the root cause then define the follow up action together.