I think when the ammonia gets to 0.25 you need to do a WC
I did a 25% water change, does it need to be more than that?
We usually recommend that people in a fish-in cycling situation perform rather large and frequent water changes (using good water change technique) until we can determine if they have (or they can get) a good liquid-reagent based test kit. A test kit of this sort is pretty much a core tool for a beginner who needs to learn about and perform a cycle.
Once you are using a good set of tests that you can trust, then you can begin to fine-tune the water change process to minimize the hard work for you but to maximize your chances of saving the fish and minimizing the amount by which the cycle shortens their lives. If done perfectly, with lots of close attention and work, a fish-in situation need hardly be too much more risky than if you had done a proper fishless cycle.
The goal, as mentioned, is to keep both ammonia and nitrite(NO2) *below* 0.25 mg/l, also written as 0.25ppm (a quarter of a single part per million.) What you're hoping to figure out is what percentage and frequency of water to change so that neither of these substances will have crept back up above 0.25ppm before you can be home to change out the water again. The straight-forward approach most often taken is to establish regular test times, morning and evening, about 12 hours apart, and always test at those times. You of course will go into it with rather large water changes, but by looking at the feedback of your tests, you can see whether you might get by changing a little less water or whether you truly need a larger water change because something went over 0.25 since the last time.
It varies greatly by how many and what types of fish are in what size tank! Less waste means and fewer fish means smaller/fewer water changes and vice versa. Usually people find that they need to try and get the results pretty close to zero ppm in order to keep them from going above 0.25ppm 12 hours later. You need to wait about a half-hour (or an hour is even better) after a water change before testing for what you acheived because the water needs to mix.
Good water change technique simply means to always remember to dose the replacement water with conditioner (Seachem Prime and Amquel+ are excellent conditioners) at the rate the product indicates or even at 1.5x or 2x the recommended dosing. While the colonies of bacteria are small and more fragile, there is a greater risk they could be wiped out by a sudden increase in the amount of chlorine or chloramine that the water authority puts in the tap water, thus the added conditioner recommendation. Don't go over 2x dosing however as it might slow some of the bacterial growth. Good technique also means to roughly temperature match (your hand is good enough for this) the return water. This is also because of the fragile colonies. In a mature tank with robust bacteria, there is much less call for this precaution unless the water change is unusually large. Of course, during fish-in cycling the water changes are often quite a large percentage!
Beginners should not be hesitant to perform large water changes if they are called for and once good technique is understood and practiced. The damage to fish from ammonia or nitrite(NO2) above about 0.25ppm is much, much more serious than most shocks they might receive from larger water changes.
Hope this helps!
~~waterdrop~~
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