How often Ultracarb filter needs to be changed?

I am wondering how often Ultracarb ceramic filter needs to be changed. It says it can be cleaned, but on the box I have, it says its capacity is 600 gallons. Does that mean it needs to be changed after 600 gallons or that it needs to be cleaned after 600 gallons? How many cleanings can you do?

The lead reduction capacity is 600 gallons, cleaning frequency depends on flow reduction. Our recommendation is to maintain optimum quality drinking and cooking water change it every 6-9 months based on household of 4 users.

I ordered a new Supercarb candle part N° W9122021 yesterday, because it seemed about time to replace it.

My question is, how does one know when the candle is used up?

Based on household of 4 person using the filtered water for drinking and cooking change the candle every 9-12 months to maintain optimum quality water.