Ketamine is not physically addictive, but, psychologically, thanks to its desirable effects and short duration, it can be extremely habit forming. There is now clear evidence of tolerance and dependence. (1)

It should not be taken if you're anything other than emotionally stable and robust. Many regular drug users are completely suprised by the "first addictive psychedelic they have ever encountered" (2)

tolerance
Bodily tolerance rises quickly with regular use and lasts for about three days.

Frequent users require increasing doses and many report a diminishing of the Ketamine high over time, so that the effect becomes more like a combination of cocaine and cannabis.

Chronic users - mainly those who inject - develop something close to permanent tolerance so, after months of use, are unable to experience the psychedelic effects. Ever again.

withdrawal
Ketamine does not appear to produce withdrawal symptoms in chronic users. (3) There are anecdotal reports of tension, twitchiness, poor attention span, and restlessness in abstinent long term users, but this may be due more to the sedative norketamine (a breakdown product of ketamine) lingering in the blood stream. (4)

1. Ketamine: Dreams & Realities p.166
2. ibid p.179
3. ibid p.200
4. ibid p.201