Purging has nothing to do with the Gain/Loss accrued on account of Investment/Trade in Equity. Purging has to be done every year, based on the available financial data in the public domain. One has to do the purging whether the shares have been held for a long time or over a short term period. Purging is cleaning of impure income accrued in the books of accounts of the company during the financial year under consideration. The longer the shares are held, the higher is likely to be the purging liability as the period during which the interest has been earned is also longer.
Allah knows Best...