The AC-30 filters are located here:
http://www.waterfilters.net/culligan/reverse-osmosis/Culligan-AC30.htm
The first stage is the sediment filter and that should be replaced approximately every 3-6 months.
The second stage is the carbon filter and should be replaced approximately every 6-12 months.
The third stage is the RO membrane and should be replaced approximately every 2 years.
The fourth stage is the smaller carbon post-filter that should be replaced every 6-12 months.
These are averages and vary based on the quality of your water and how much water you use.
Thank you,
WaterFilters.NET