Yep, it has a strong "urge to redose" side to it.
Don't buy more at one time than you will do in one weekend.
That way, once it's gone, the urge to re-dose again and again is gone before you can score more, or in my case I HAD to stop until my next order arrived from Europe. So, in effect, I was limited by the postal service and my vendors supply. Better to pay shipping on one vial every week or two than to "save" money and get 10 vials at once and end up doing all 10 in one binge.
For me, Ketamine had a stronger psychological Jones than heroin or any of the myriad of other shit I've been addicted to over the years. Thankfully I built a tolerance up to the point that it wasn't fun any more so I stopped a couple/three months ago...knock wood.
So, the moral of my story is WATCH OUT!!!
OP, I just realized I didn't address your actual question: "How often can you safely do Ketamine?"
It's a little like the question, "how many licks does it take to get to the chewy center of a Tootsie Pop?"
The answer in both cases is it depends on you. If you can't quit at will then once is too much.
I know, I'm still dancing around the question, mainly because I don't KNOW how much you have to do to incur actual physical damage to your body...that also depends on you...general health, genetics, etc...
That sure took a long time to say "IDK", huh?