For OpenAI, it seems like you can't prolong the caching duration for money. Duration is longer during off-peak hours for in-memory caching and up to 24 hours for extended prompt caching. https://developers.openai.com/api/docs/guides/prompt-caching
For DeepSeek, caching duration of at least 12 hours (and likely longer) have been observed. Cache writes are free. https://zhuanlan.zhihu.com/p/2035737726952194774