The terms of conversion of convertible preference shares are defined at the time of application itself, for example, how many shares is the preference share convertible into, the specific time period for each conversion and the price (Either fixed or tentative). As an investor, you cannot convert the preference share into equity shares whenever you want.
For example, a convertible preference share of Rs 200 may be converted into 2 shares, one at a price of Rs 75 at the end of 12 months and the second at Rs 125 at the end of 24 months. The investor cannot ask the company to convert the preference shares at dates or terms other than these, they are automatically converted as legally defined here.
There is a specific category called optionally convertible preference shares, typically issued only to select corporate investors/ promoter groups, where the investor has the option of converting the preference shares into equity at a certain price and time, or continuing to hold them as preference shares till they mature. The optionally convertible preference shares are typically not offered to retail investors