Anthropic's cache expires after 1 hour when using subscription endpoints, and for those cached tokens cache reads are free. It's generous (compared to API pricing) but it's not 100% free.
I believe it's 5 minutes on API pricing by default, though you can turn off caching or force it to 1 hour. Subscriptions are special, seemingly because Anthropic doesn't want to expose casual users to all of the tokenomics.