Well that depends on what you mean by completing a fish in cycle. But lets start with the fact that most experienced fish keepers these days do no really know the proper way to cdo a fish in cycle, you will see a lot of what I consider to be be bad answers. So I will give you two answers to you question and do so under the assumption that one does all parts of a fish in cycle properly.
There are two possible ways to answer your question. One is to answer how long it takes to complete all the stages of a multi-stage cycling process to end up with a fully stocked and completely cycled tank. This should take between 12 and 15 weeks. The other answer is how long does it take to complete the initial stage of the process which should take about 35-42 days. These times are on average and may go faster or longer.
The best way I know to explain a fish in cycle done properly isthat it is a dance between allowing ammonia and nitrite levels to rise as much as possible for the cycle and not allowing those levels to get high enough to do any permanent damage to the fish. What it takes to perform this dance is a lot more complicated than what it takes to cycle without fish. One thing that is needed to maximize the speed and to avoid the unneeded stress to fish from lots of very frequent water changes, is to minimize their frequency and amount. In a fishless cycle the only concern is what will get the tank fully cycled to handle a full fish load the fastest. What the bacteria need for this is not so safe for fish. And this is the big difference between the two forms of cycling. Fishless gets you ready to stock fully in the same or less time than just the initial stage that fish in cycling can. Plus no fish can get hurt or killed if one makes a mistake.
That means one needs to be able to test not just for total ammonia but to know how much of it is in the toxic NH3 form vs the nowhere near as harmful NH4 form. (have a read at the SeaChem site
http
/www.seachem.com/Library/SeaGrams/Ammonia_Management.pdf ) One needs to have the proper number of the right species of fish since some are way more ammonia tolerant than others. of course one also needs to start with healthy stock since going through a cycle is stressful to fish, even when done right.Weak fish are less likely to survive.
My advice to you is, rather than try to do a fish in cycle, return the fish to the store for credit so you can come get them back and then complete the cycle as a fishless. Or ask a friend who keeps fish to hold them for you if possible. The other choice is to add enough bacteria to the tank to get it cycled or very close to. You can do this with media, substrate or decor from cycled tanks and/or bottled bacteria such as Dr. Tim's One and Only or Safe Start from Tetra. Be sure to follow the directions for these bottled products to the letter. Both have sites with more information.