Cocaine is highly addictive and an astonishing waste of money. You also come across as attempting to normalize or explain what is probably already an addiction. I have heard the same lines from dozens of people experimenting with addictive drugs.
The main danger is cutting agents as these can include glass, crushed caffeine pills, pain-killers, amphetamines etc.
Common cocaine additives include:
Local anesthetics like procaine
You can also experience serious heart failure if you exhaust yourself during the high, this can be in a club, going for a run or during sexual activities.
Cocaine will definitely cause serious heart problems if you take up the habit frequently and some people die in there 40's because of this.
Mental health issues I have seen associated with cocaine abuse are by far some of the worst of any drug. You can also expect it to cause brain damage, schizophrenia and paranoia, memory issues, anger management issues, inflated sense of self and ego and you are far more likely to endanger your own life while high.
Short term effects;
Mixing cocaine with alcohol is 5 times worse for your health at least, according to most experts. Similar to a Marijuana use mixing the buds with tobacco leaf. So all of the above, but around 5 times worse.
Other harmful effects of mixing alcohol and cocaine can include:
The risk of cocaine overdose can be increased by mixing it with alcohol.
Intracranial haemorrhage (bleeding in the brain).
Chronic cardiac toxicity.
To answer your question, If the product is high quality maybe half a gram a week is reasonably safe if you are not drinking or physically exerting yourself. I would be careful though, it is unlikely but theoretically one line could kill you. It is also worth noting if you are offered a line by a stranger you should refuse it as it could be spiked with a date-rape drug and you could be robbed, raped or murdered. Just don't trust people on this drug.
Dangerous levels I would say if you ever begin using every day or two nights in a row. And if you ever get the urge to sprinkle it on your cornflakes. Then you have a 'Cocaine problem'