Google's support for the Chromebooks is at least five years from the initial hardware release but may sometimes be extended with a notice. Here's a full write-up from the Google support page on their policy, the link below also provides a list of devices with specified end of life dates.
•When a device reaches End of Life (EOL), it means that the product model is considered obsolete and automatic software updates from Google are no longer guaranteed.
•Work and Education customers using devices that have passed their EOL date will no longer receive full support1 from the Google for Work and Education team.
•Work and Education customers using devices that have passed their EOL date may find that they cannot manage their devices as expected using the Google Admin console or leverage new management features released.
•For Android tablets for education, the ability to receive content sent from Google Play for Education and a new user account through device setup is not guaranteed on models which have passed their EOL date.
•Google will provide advanced notice of a model’s EOL Date on this page as soon as it is launched, giving our customers time to make purchase decisions.
•EOL dates may be pushed later than the initial date published, but will never be sooner than listed, which will be at least 3 years for Android tablets for education and 5 years for Chrome devices from launch of the hardware.
•Android tablets for education models that have not reached their EOL date will continue to be able to be set up and receive content pushed from the Google Play for Education site, as well as be managed with the Work and Education management service.
•Chrome device models that have not reached their EOL date will continue to receive OS updates and function with the Work and Education management service.
•The End of Sale date is controlled by the OEM (manufacturer) of the device model and has no relation to model’s EOL date. Therefore, it will not be shown in the table."