How long are generic product keys good for?

Generic keys work for installation but not for activation. If you don't activate you're automatically on a trial period of either 30 or 90 days, after which the OS becomes increasingly unattractive, harder to use, and reminds users more frequently to buy a license or otherwise activate it legitimately. @Brink has a nice TenForums tutorial on generic keys: Windows 10: Generic Product Keys to Install Windows 10 Editions. It does not address the trial period's duration, though...
HTH,
--Ed--

The link @jkirkman referenced is broken, the final ')' gets removed from the link by the board software. This link works...

Microsoft Docs | Appendix A: KMS Client Setup Keys

Key Management Server (KMS) generic keys should only be used if you are installing Windows on a PC that will be used in an enterprise environment where volume licencing is regulated by a Key Management Server. A KMS-activated PC has to regularly keep in contact with the KMS to renew its activation. Volume licencing is not available for personal use. I believe KMS activation expires after 90 days.

Any 'hack' to make KMS activation work longer than that is illegal and against forum rules to say how that can be done. Anyone who buys a PC which has a KMS key is the victim of a scam, they have been sold a PC that has had pirated software installed. Unfortunately this is common practice in the shadier corners of eBay.

The Retail generic keys do not expire, but they don't activate Windows either. It is also a violation of the EULA to use an unactivated copy of Windows.