Sorry guys and gals... there is no standard answer nor any magic algorithm that can tell you how often you should train Brazilian jiu jitsu. Everyone's life is different, and everyone's jiu jitsu journey is his/her own.
However, there are a few questions you should ask yourself to figure out how often YOU should train BJJ. Read on...
How much do you want to?
How often you should train will be dictated largely by your will; how much do you really want to train? Are you absolutely obsessed with the gentle art and want to train every day, all day, and only that for the rest of your natural born life? Or do you just love it and want to hit the mats a few times a week to get your adrenaline and endorphin fix, in between the days you do other things that you love? Either is fine, and completely normal, believe me. What's most important about this question is that you can answer it honestly and without any outside influence. Make sure you're training only as much as you want to, not as much as someone else wants or expects you to. When it becomes about what someone else wants, you'll quickly find your will to train deteriorating... and nobody wants that.
How much can you?
That first question will be significantly tempered by how much you can train. Maybe you have a full-time job and a family. Even if the will to train every day is there, your available "me time" is just not there. You have obligations, which is cool. Congratulations on being a successful adult. But because jiu jitsu is such an excellent vehicle for relieving stress, regardless of how busy you might be in your real life, you should make the time to hit the academy a few times a week... for your own health and sanity (which only has positive effects on your family and work relationships). Or maybe you do have the time, but you have physical limitations on how hard and often you can go. As someone in my 30s, I know that my ability to recover is not what it used to be, so instead of breaking my body to keep up with all these superhuman kids, I train less, but train smarter.
What are your goals?
Then you have to factor in your goals. Do you want to be a world champion athlete? You're not going to do that by training just a few times a week. There are people out there busting their butts each and every day to make it to the top. You're not going to get there by cruising. Do you just want to stay in shape? You probably don't need to train seven days a week (unless you want to... or like me, you have a terrible addiction to sweets); four or five days a week is fine. Do you want to become a black belt in 8-10 years (that's pretty fast, btw)? Then you should be training often and consistently. If you don't really have any goals, and you're there just to have fun, you can train however often you feel like it.
How do you feel?
Which brings me to my final consideration: how do you feel? However often you decide to train, you should always listen to your mind and your body. Have you been training six to seven days a week, hard, and you feel like your 90-year-old grandpa when you get out of bed? Then you should probably tone it down a bit. Are you forcing yourself to get into your car because you feel like crap and you kind of dread going and getting smashed on? Don't go. If your mind and body aren't feeling it that day, you may not be able to perform as well as you can and you might get hurt, or worse, burned out. Of course, there have been times that I've felt a little "meh" and forced myself to go train, only to feel 100x better afterward and glad I did it. But there's a fine line, and you should make an effort to recognize where that line is for you.
However often you train jiu jitsu, remember to always have fun.
Train on... often. Oss.