One to two months' rent is the global norm, with the exact figure set by local law or custom: some countries cap deposits legally at one or two months, others customarily ask two or three, and a few markets are famous for heavier upfront demands. Furnished places often command a higher deposit than unfurnished. If someone asks for far more than the local standard, that itself is worth questioning; check what's normal in your specific country before agreeing.
Refundability follows the same logic everywhere: the deposit returns at move-out minus legitimate deductions for unpaid rent, unpaid utilities, and damage beyond normal wear and tear. That last phrase is where disputes live, since scuffed paint after two years is wear and tear while a broken door is damage, and landlords vary in how aggressively they interpret it.
Protect yourself from day one. Photograph and video every room, existing scratches included, at move-in, and get a signed inventory or condition report.