Hi Wirriam,
Frequency is roughly 6000L or 1 year based on what was advised. It's like servicing your car etc... to save my bucks.... I would go for 6000L (which the system will actually prompt you)
You can change it yourself, but you don't need to sweat a muscle... its free delivery and changing... just need to order and buy the filter.
Changing for filters depend on what system you are using... my old Jupiter Delphi filter cost roughly $150... The newer Smart I models are slightly more exp... there is a new material in there called KDF which bumped up the price)
Hope the above helps!
Cheers!