Around has given you some good advice.
You might consider some sort of formal schooling to help prepare you for the exam. For example, there is NPTI, National Personal Training Institute, that has a 500 hour program of both education and practical experience training your fellow students. It’s 500 hours of curriculum plus homework / study, and depending on whether you go to school during the day, at night, or on weekends (they have flexible programs), it can take 6 months to a year. Two of my friends have done it and were pleased with the results, and I’ve had about a half-dozen coworkers who were hired post completion of the program. At the end of the course, most people are ready to sit for the NASM exam. With the hands on experience you get training, it’s easier to get a job, and I’d advise starting at a gym that lets junior trainers shadow with experienced trainers.
As for me, I was already in group exercise with several group-ex certifications. I sat for the ACE exam and I studied 2 hours a day for about 3 months. About 200 hours. It was just about right. I passed with much higher scores than I needed, but I wouldn’t have studied any less because I didn’t want to sit for the exam more than one time.