How many days totally depends on how much you take, and in what timespan.
If you didn't know: drugs have a half-life, which means that there is a certain time for half to be broken down in your body.
The half-life for ketamine is 2.5-3 hours which means that if you start clean, then take 160 mg of ketamine (assuming that you administer it with 100% efficiency like injecting), 2.5-3 hours later there will be 80 mg in your blood, another couple hours later 40 mg, then 20 mg, etc.
To make matters more complicated, ketamine is broken down to norketamine which has a longer halflife and may also be detected (I am not sure).
And from the blood these compounds are filtered by the kidneys and pissed out (among other routes of excretion), this again happens at a certain rate I am not sure of.
So people giving you a number of days are probably considering some kind of standard or maximum reasonable amount of ketamine taken, then calculate an average amount of time to get it removed from your body, they might add a day just to be 'sure', who knows.
7-14 days is more reasonable if you analyze the theory I just gave you, plus giving a large range is much more realistic than just a single number.
In conclusion: though you did not say anything about it yet, let me just say that I would not try to calculate exactly how much you can take and when, pushing the envelope, because it seems too complicated with unknown factors like rate of excretion and dare I say foolish typical behavior of an addict (no offense, I was a K addict myself but am recovering). So don't make it hard on yourself and allow for ample time before a drug test - or otherwise get prepared to deal with possible consequences.