It depends on the type of car such as whether it's a front wheel drive car or a rear wheel drive car.
I've changed a rear wheel drive cars transmission out myself and it took me 2 days to complete the job.
And then I attempted a front wheel drive cars transmission change and it took me nearly a month to get the job done.
A front wheel drive car needs to have the body lifted away from the engine and transmission to access and change out the transmission where's on a rear wheel drive car you can just separate the transmission from the engine a lot easier.
Now a transmission shop with some more help may be able to change the front wheel drive transmission out in a week or two or even much quicker but if you're doing it yourself and it's your first time expect to take a month or more to change the transmission out.