Author here — great question, and you've hit the exact reason the calculator
uses an adjustable cache-hit ratio (default ~70%) instead of assuming 100%:
real hit rates depend on session gaps, and TTLs differ a lot by provider.
Rough current state: OpenAI's automatic caching typically keeps prefixes warm for ~5–60 min depending on load; Anthropic's default cache TTL is 5 min (extendable to 1h for an extra write fee); Gemini's explicit caches let you set your own TTL with per-hour storage pricing. So your 20-minute coffee break is usually fine on OpenAI, a cold cache on default Anthropic, and whatever you paid for on Gemini.
Adding an "average gap between messages" input that derates the hit rate per provider is a really good idea — putting it on the list. Thanks!