Susan, depending what you use, I'd say a good two weeks, max. What are you using to resolve this problem? Toni

Actually no one can answer your question with any type of accuracy, because you do not give enough information. Was the chlorosis due to a lack of nitrogen, or was it due to a lack of iron, or magnesium, or zinc or a combination of deficiencies? It is also possible for a chlorosis be caused by over watering. Some causes of chlorosis never return to a healthy green leaf color. If you want a knowledgeable answer, we will have to know what the cause of the trees chlorosis resulted from - Millet

I think it was due to the ph being too high. They are small potted plants. So I have added a soil acidifier, but in the meantime, I am adding vinegar to the water. Hope that doesn't sound silly. It worked before on another type of plant, but that was a long time ago, and I don't remember how long it took.
I've been really careful about the watering, and I'm sure it's not that. And it's on the new leaves only.

Susan

When the pH of the growing medium increases to the higher ranges, iron, manganese, zinc and boron become more and more unavailable to the plant and chlorosis is usually the first symptom. Nitrogen deficiency shows up on the older leaves first, with the newer leave still showing some green, so we can rule out a nitrogen deficiency. However, Iron and manganese deficiencies both show up on the new leaves first. Iron, manganes, magnesium and zinc deficiencies are often caused by high soil pH, rather than a shortage of the minerals. Vinegar will lower the soil pH, but does not work very long AT ALL, because vinegar is an organic acid. Usually a mineral acids are used. Normally soil pH is regulated with the use of an acid based fertilizer. Lastly, ALL NEW LEAVES of citrus are ALWAYS a light pale green color when new, then darken with age (aprox 4-6 weeks). It could be the normal new leaf green coloration and actually not a mineral problem at all. Take care. - Millet

Great info, thank you Millet. Mineral acids...never heard of them. Where might I get them? I do use an acid based fertilizer, but I have a "light" hand. I am only using the vinegar as a temporary fix until the soil acidifier starts working (hopefully)
The yellow leaves don't seem like healthy new leaves at all, but the rest of the plant looks good. Only about three leaves are affected. There is one lemon growing, slowly, but that's more than I had hoped for. This little tree is in my greenhouse. The smaller tangerine, and the bigger calomondin and grapefruit are doing fine. The grapefruit is an ugly tree, but it has fruit on it and it's very hard to get any kind of selection of citrus trees at all up here in Canada.
I'm sure I'm not overwatering, as I have learned that lesson in the past. When I do water, I use a nice temperature of water.
I just love these plants.

Susan

You have heard of mineral acids, you just don't know the term - Sulfuric, nitric, hydrochloric and acetic acids are the mineral acid. Take care. - Millet

Susan, vinager will acidify the soil. Perhaps you should buy a guage..they start at 9.99 and up. I use Rapitest brands..How much vinager are you using.>I've read 1 capful to a gallon of water..does that sound about right to you? Toni