It depends on what type of fish it is, but it usually takes 2-4 months, depending on the type, the water, the parents, the amount of food, but in general 2-4 months.
Professional breeders let their guppies grow till they are about five months old. Males and females must be grown separately because females may get pregnant very early, sometimes when they are only three weeks old. Then they keep on growing, and, of course, they will not give birth to their fry until they are big enough themselves, but the problem is that a female that got pregnant very early is not able to grow up to its full size. So, you should better separate males and females as soon as possible. You may normally tell the sex of young guppies even when they are only two weeks old. Males become a little shiny and their anal fins start turning into a kind of tube.
Depending upon the fish. Some fish grow to full mature size in a few months.A swordfish is full grown at 2 inchs, a neon at 1 inch. If it is a gold fish it will only grow as big as the space it has will support. For instance a 1 in gold fish placed into a ten gallon tank with minimum extras will reach it's support length in 3 to 4 months as long as it is well fed. You could have him in this tank for a couple years and then place him in a 30 gal and he will grow bigger. You could also take that 1 in gold fish and place him in a tank with 10 other fish and he will not grow at all. You could purchase an oscar at 2 inches place him in a 30 gal long and have a 8 inch fish in 6 mths. As you can see size of your fish will depend upon his natural mature size, the feed, and tank size.
I need a little more information to be able to answer your question accurately.Specifically, what kind of fish do you have?Various breeds will have different life cycles.For your typical goldfish they should be a decent size by about six months.They should reach adult maturity and be able to breed again by the time they are 2 years old.
It depends on the kind of fish it is, the size of the aquarium, the foods he eats and how many other other fish occupy the same environment. There are many variables on this one. I could ask my brother, he is not currently on the site but has knowledge on this subject. Auntimaime-
