There are several ways in the process of burning coins, the ways are varied and determined. Some developers, will use one step burning after the ICO is over to remove unsold coins from circulation.
Then, there are also those who prefer to do it in several stages. Just like binance which does the coin burning process every three months, this burning is part of a commitment to reach 100 million BNB which will later be burned.
this is more rational than burning when the coin is not developing, although burning is considered important for the development of coins in the future, the coin burning process is based on a study or analysis of the pre-sale circulation, usually they do it when the ICO has been completed and delete unsold coins, or as you say using the burning process every three months after going through the existing mechanism.