From the US Patent Office
https://www.uspto.gov/patents-maintaining-patent/maintain-your-patent
What happens if I don’t pay a maintenance fee?
If maintenance fees and any applicable surcharges are not paid the patent protection lapses and the rights provided by a patent are no longer enforceable.
Mailed notices
It is the responsibility of the patentee to ensure maintenance fees and any applicable surcharges are paid timely to prevent expiration of the patent. If the maintenance fee is not paid within the first six months in the year in which it can be paid, a Maintenance Fee Reminder notice is sent to the fee address or correspondence address on record.
If the maintenance fee and any applicable surcharge are not paid by the end of the 4th, 8th, or 12th years after the date of issue, the patent rights lapse and a Notice of Patent Expiration is sent to the fee address or correspondence address on record.
Failure to receive the notices will not shift the burden of monitoring the time for paying a maintenance fee from the patentee to the USPTO. If a fee address has not been established, the notices are sent to the correspondence address.
Note: there are numerous companies unaffiliated with the USPTO that send solicitations to patent owners concerning maintenance requirements. See the warning about non-USPTO solicitations for more information.
Expiration dates
Generally, utility patents expire after 20 years from the application filing date subject to the payment of appropriate maintenance fees.
The USPTO does not calculate the expiration dates for patents. In response to patent owner and public inquiry, the USPTO provides a downloadable patent term calculator as a resource to help the public estimate the expiration date of a patent. See the Patent Term Calculator page for more information.