Quite some time ago I installed one of those 5-stage water filtration systems to filter (pattaya) city water for coffee/tea, ice and cooking use so that no need to carry anything other than drinking water (that I very rarely use) . Only use the filtered water either boiled or frozen. 

 

Wondering how often I should replace the filter cartridges. Wait till they get blocked, or should I do it regularly?

 

I use something in range of 2-4 liters per day or something like that. 

Most claim 5000 litres.

At 5l/day that's every 3 years.

Although I've noticed the filters cost more than the original unit, so chuck and replace.

Not sure of the system you have but for my RO it is recommended to replace all but the RO every year and the RO at about 1.5 or 2 years.  First filter is probably best to change every 3-6 months as that removes most of the <deleted> (and in many units is in clear casing to remind you to change it when dirty).

lazada sell them cheap,Tesco too

I change my filters at some point after 6 months, I monitor the output flow and make a judgement based on the flow, this could be even be 8 months between change. 

I buy online from Duan & Daw in bangkok. Last purchase was

1 Ceramic 

1 carbon

1 resin 

Bhat 440 + plus a minimal delivery charge - 2 / 3 days from order. 

 

Frequency for change depends on incoming water quality - so there are no hard and fast rules. 

 

15 minutes ago, Artisi said:

I monitor the output flow and make a judgement based on the flow, this could be even be 8 months between change. 

PPM pen about $10 from Lazada.

My Chiang Mai tap water is 80ppm straight out of the tap.