What is the ISP Log retention timeline?

So I have this particular question, how do ISP store Ip logs in timeline terms?

Suppose an ISP stores logs for 6 months.

Do they store the january-december (Yearly) logs for 6 months,

Or do they rotate so each month is only stored 6 months.

Example: January logs is stored for 6 months, and then gets rotated, then February log gets stored for 6 months and then gets rotated, and so on.

I hope to hear from you guys Best Regards