How long can a company be called a 'startup'?
An entity would cease to be a 'startup' upon expiry of:
a) 5 years from the date of its incorporation/ registration, OR
b) If its turnover for any of the financial years has exceeded INR 25 crore; OR Startups would be required to intimate DIPP of any such cases within a period of 21 days.
Frankly speaking, those are the government definitions. A startup should stop calling itself a startup when it's found it's business domain and a working product-market fit, and has began establishing processes for business operations.