Indicator of water change

Personally, I feel you're better off to be proactive with your water changes. If you wait for numbers to rise, you are causing fluctuations in your water chemistry. It should be better for your fish if you find a frequency/volume that keeps your params consistent and stick to that.

The Eheim will show that it is in need of a good backflushing or teardown/rinse/repack (see www.rexgrigg.com for a well written procedure) when its flow drops off a bit.

Regular water changes should be done regularly as "stuff" will build up over time. Most FO tanks work out w/ 25% every other week - % and/or frequency are a function of the number and size of fish.

With a freshly cycled tank, add some fish and keep testing frequently, especially for NO2. If you see a spike, do a water change to keep NO2 below 0.25ppm (below 0.1 is better still) and test again the next day and the day after that. If NO2 is stable at ~0ppm, you can gradually add more fish until you reach your planned stocking level (you do have a plan, right?).

I find it easiest to do water changes in a routine(same day every week this way I don't forget). I do weekly water changes of 20-30%.

I believe water changes and when you do them is on a tank by tank basis. many just choose to do weekly water changes while others may do more frequent changes.
part of the decision is based on stock levels part on species.

some fish need pristine water while others are fine with weekly or even every other week.

higher stocking levels will need more water changes as well as determining the needs of the species(pleco tanks will probably do better with more than 1 water change per week).let alone Discus(especially juveniles) blue rams etc...