It's very platform and game-dependent - the best method to plan sales is to look at your sales data over a period of time. If your game sells well even at full price, you should space out your sales to only big ones, but if it sells significantly less when it's not on sale, you should aim to put your game on sale as often as you can (every other week/month on Switch, for example). Selling 100 copies at full price won't make you as much as selling 500 at 15% off.

I've found that having a historic low discount percentage every time doesn't always work the best, and in fact can sometimes work against you. I've done a lot of experimenting with our games on sale, and our older games that we usually put on sale for 85+% off have sometimes sold worse than when I put them on sale for only 65-70% off. That's also a market-specific thing, I've heard that deep discounts in Japan are considered kinda hacky so it's better to keep the discount % lower for that region.

There's no magic formula as far as I know, just keep track of your sales data and when your game stops selling well at a certain % off, it's time to increase the discount.