On a standard German tenancy the deposit is capped at three months' Kaltmiete and you are entitled to pay it in three monthly instalments. It is supposed to be held separately from the landlord's own money and to earn interest, which is yours. On short sublets the practice is looser — one month is common, sometimes none at all — but the three-month ceiling is a useful sanity check against anyone asking for more.
Now the part that matters more than the law. Every large-scale Berlin rental scam is a deposit scam. The shape is always the same: an unusually good flat at an unusually fair price, a plausible reason the owner cannot show it in person — they have moved to another country, they are a doctor working abroad — and a request to transfer the deposit before you view, sometimes dressed up as an escrow service or a key-courier fee.
The rule that defeats all of it: never send money for a flat you have not stood inside, and never to someone you have not met or at least spoken to on video. There is no legitimate reason to pay before a viewing. Any explanation offered for why this one is different is part of the scam.
When you do pay, pay by bank transfer with a reference naming the address and the word Kaution, get a written receipt, and photograph the flat thoroughly on the day you move in. Getting a deposit back from a short sublet is a matter of the other person's goodwill far more often than it is a matter of law, and photographs are what keep the conversation friendly.