The time limit  will be fixed by the Deputy Comm. Income Tax while sending the notices. It may be by week, or a month and it all depends.

Normally a reasonable time has to be granted to respond to a statutory notice under the act. Like Notice u/s 142(1), 148 etc. However, regular IT hearing notices are issued depending on the compelxity of the data and details called for in the notice. Once the proceeding gets underway, notcies can even be issued for next day.

Sometimes such notices are issued to harass assessees and representatives and intimidate then into agreiing for an addition. But if the notices are unreasonable, it is better to ask for an adjournment or for some more reasonable time prefereably in writing - especially if the notice is issued in writing.