How many months
The security deposit cannot exceed 3 months' rent (usually it's 1–2).
It must be stated in the contract and bears no interest unless the law provides otherwise.
When it's refunded
On return of the keys, once the absence of damage beyond normal use and payment of all sums due have been verified.
Always make a handover report with photos at move-in: it's your best protection against disputes at the end of the contract.
Booking deposit vs security deposit
The confirmatory booking deposit is paid at reservation and has different effects in case of withdrawal; the security deposit guarantees obligations during the lease.
Be wary of anyone asking for large deposits before showing you the property or signing: it's a possible scam signal.
Informational material by casaperstudenti.it: it is not legal advice. For your specific case, consult a qualified professional.