Every supplier of goods is required to take GST registration online if his aggregate turnover exceeds 40 lakh.
Documents for registration - PAN card, aadhaar card..There are no time restrictions given in law for the validity of the GST number.
Registration is valid for a lifetime if It has not canceled by the tax officer nor surrendered by the taxpayer.However, In the case of a casual taxpayer or non-resident taxable person, the validity of registration is limited for a period of 90 days or the period mentioned in the registration application.