The reward started at 50 BTC per block in 2009 and dropped to 3.125 BTC at the April 2024 halving. The next one is expected around 2028. Halvings will continue until all 21 million bitcoin are mined, around the year 2140.
Past halvings came before big bull markets, which created the idea of the four-year cycle. Whether that pattern still holds is debated.