How Often Does Bitcoin’s Difficulty Adjustment Occur and Why?

Bitcoin's difficulty adjustment occurs approximately every 2,016 blocks. Since the network aims for a block to be found every 10 minutes, this corresponds to roughly every two weeks.

The adjustment is necessary to maintain the 10-minute block time average, regardless of the total hashing power entering or leaving the network. If blocks are found too fast, difficulty increases; if too slow, it decreases.