In order to be able to use your card at Point Of Sale machines, you ought to activate it as soon as probable using any of the means that the credit card offers. However if you have been caught up and could not activate it instantaneously, there is no requisite to concern. Typically, there is no time limit to activate the card but for your credit card provider has identified if not. On the other hand, the OTP sent to your number will be valid only for a restricted period as stated.
As a result, if you have a credit card for to a certain extent some time and have not been able to activate it, you should call the customer care service to generate a new OTP and use that particular to set your first-time PIN.