Some people check their nitrate levels and gauge when to make their water changes when the nitrate level gets around 5 or 10. Most people I know just do a weekly gravel vac which also takes care of your water change once a week whether it needs it or not. I use a python, so when I am adding water to the tank, I usually pour in the dechlor and then point the hose where the dechlor is. If you mix your dechlor into your water in a bucket before adding it to the tank, as long as the temp is good you can add it to the tank immediately.