What will be the duration for which telecom company blocks my iPhone?

There usually is no time limit for it to unlock. You need to contact the carrier it’s locked to, assuming your carrier if you want to have it removed. In most cases there is still a contract or agreement in lease that is still going or unpaid. But if you happen to purchase this iPhone used, you were likely scammed with a stolen iPhone and you are best off returning it for a refund. Only the carrier it is locked to can unlock it, and they only do so when you are in good standing relation with them (the original owner that signed the contract), and the lease paid off.

That is entirely dependent on the carrier it’s locked to. You’d have to reach out to them to find out what their terms are, including the time frame to unlock it.