I am getting married can I pay my Maher amount after the marriage or do I have to pay on the spot? If I fail to pay what will be the punishment? Till what time can i pay that to my wife?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

The Mahr (bridal money) is one of the obligations that should be fulfilled in the marriage contract. It is not a condition that you name at the contract or you pay during that ceremony. It is subjected to the agreement of the concerned people. So, it might be given before the contract, during it, or after it, according to the agreement.
If the person is unable to pay the Mahr, it remains a debt on him until he becomes able to pay it unless the bride waives it or drops part of it as she is the one who has the right to do so. Allah says: {…..but if they, of their own good pleasure, remit any part of it (Mahr) to you, take it, and enjoy it without fear of any harm (as Allâh has made it lawful).} [4:4].
Allah knows best