when you say ‘fitness instructor’, do you mean personal trainer or group exercise instructor? Those are two different certifications, and one does not contain the other.
The length for those course varies widely. I am sad to say that there are still “certifying organizations” which will give you a piece of paper with either one of those titles just after a weekend’s time investment. I am not saying that you will not be learning anything during that weekend but I do have an issue with that because the entire industry gets a bad reputation because of it.
If you look at the ‘Career Guide’ on this website, you will find a myriad of organizations that certify. My recommendation is to go with an organization like ACE, ACSM or NASM (other people will recommend different ones) which have stood the test of time in the industry and are in high regard.
The length of those courses depends in part on you. The personal trainer certification from NASM, for example, gives you a year during which you must take the test. You will have an option to go to a seminar and will receive an extensive textbook.
I hope this helps, and I wish you the best of luck.