When a mains supply is supplied through relay, how much resistance the relay contact is likely to introduce

The electromechanical relay has inherent resistance offered when current passes through it. The question is how much the resitance in ohms must be and will the resistance increase as the contact is used over years ?

0.01 to 0.1ohm depending on the current load requirement. Contact surface resistance keeps on increasing if on and off happens quite often until contacts burn out in months or years.