How often are AI engines actually updating their index?

I keep getting asked this in customer conversations and don't have a great answer.

From what I've measured directly (not from vendor PR):

• Perplexity: roughly weekly for high-signal sites, can be days for fresh well-structured content. Reddit/HN posts get pulled almost real-time.
• ChatGPT (browsing mode): 2–6 weeks for new pages to show up in citations. Brand-name probes lag longer.
• Google AI Mode: tied to Google's regular crawl cadence, so days-to-weeks depending on site authority.

This matters because the 'wait two weeks then re-scan' rule of thumb is too short for ChatGPT updates and too long for Perplexity.

Questions:

• Does anyone have systematic data on this?
• For high-cadence niches (eg. AI tooling, where the landscape moves weekly), are you tracking changes more aggressively?