If it's 50mg/ml, that's 1000mg a week. The big problem is that the damage is slow and continuous... at that level, it's like going out and getting hammered on booze once or twice a week. It won't really do you much obvious physical harm in the mid term (years) but decades of it will cause you some very serious long term health problems. Keep a VERY good eye on your bladder and urinary tract, that's where the physical damage is going to show up, as well as your nose potentially. If you're getting abdominal pains, passing blood in urine, urinating much more or less than usual, take it easy. For long term use (and always assume its long term use) I'd try and keep it under 1000mg a month. Don't worry, you're not zooming towards imminent bladder removal or anything, just keep the use reasonable, and stop completely if you get any urinary problems. You'll certainly be at a higher risk of urinary tract infections (which are generally harmless but scary if you use Ketamine), so try your best to keep your bladder healthy. Trust me, UTIs are unpleasant... Drink PLENTY of water, lay off the alcohol and caffeine intake. Your urine should never be dark yellow, only slightly so in the morning... if your piss looks like Lucozade Original, you're massively dehydrated, so do something about it :D
Purely from a harm reduction point of view, you also have to consider the route you take the Ketamine. There are no studies to prove this with Ketamine, but with many other substances it's the AMOUNT you take that matters. If you're taking 250mg insufflated to achieve a K-hole, you're doing double the damage to your urinary system than if you were to take 125mg via intramuscular injection... yet getting exactly the same effect (just with a different time scale), because IM has a far greater bioavailability than nasal use. If you've got pharmaceutical grade Ketamine, IM is a possibility and as long as it's done properly with alcohol swabs, brand new needles and syringes every single time, you're only doing the damage of an IM injection (which is fairly minimal, many very unwell people have thousands a year for life with little issue, it's nothing like IV at all). You're certainly doing more damage to your nose than you'd do to your deltoid muscles with properly performed IM. I'm certainly not advocating IM use because it's absolutely not for everyone (and even less so if you've not got sealed vials), but just be aware than you're using less to get the same effect, and in my humble opinion, the IM experience is far superior.
And of course, keep your mind... in mind. John Lilly did heroic amounts of Ketamine over his lifetime, and apparently suffered no ill physical effects (besides falling of a bike while high on another experimental dissociative, and nearly drowning in a hot tub while on Ketamine). And he wasn't exactly young either, the older you are the less resilient your urinary system is, the elderly very frequently get UTIs. John Lilly DID however have what some people would describe as periods of serious mental health issues because of it. Others believe every word he said was true, but that's irrelevant. Just remember that your mind will probably start to suffer long before your body will... pay close attention.
