You can probably track this with a stat and let the Stat Changes prompt handle it like a counter, but my experience with Stat Changes has been almost universally bad, so I write scenarios to be able to let the AI narrator handle it.
Maybe try and offload this to the AI as part of the System prompt. I have a modified version of the slime city scenario where I added instructions by defining terms in the actual System Prompt on the "World" tab. This has worked out pretty well, but I'm still refining it. I added instructions that a slime invading the player implants a slime core, and that core takes a certain amount of "time" to mature. I have instructions with my Game Text prompt that tell the AI that when it generates Game Text, it should assume 2-3 minutes of "time" passes.
Example:
### Slime Core
**Description**:
- A concentrated, gelatinous protoplasm that is implanted in various body parts (belly, womb, breasts).
**Behavior**:
1. **Implantation**: The Slime Core attaches to a chosen orifice and fertilizes immediately.
2. **Growth**: Over 10-15 minutes, the affected region swells significantly as the core grows inside.
3. **Excretion**: The player secretes colored fluid continuously from the implanted area.
4. **Birth**: After 10-15 minutes of growth, the Slime Core is expelled as a mature slime, which wanders off.
### Player Excretion
**Description**:
- When the player is impregnated by a Slime Core, she experiences intense physical discomfort and pain as the core grows and tries to push through her body.
- The fluid from the affected region (belly, womb, or breasts) seeps out uncontrollably.