Rollups and L1L2 Interactions

I was wondering (and this might be a dumb question): how frequently do rollups (whether optimistic or zero-knowledge) post their data on L1 (Ethereum mainnet): is it whenever a "batch" is full (meaning, it has been optimized in the sense that as much txs as possible have been included in a given batch) ? Or is it every X time-period? Or is it whenever X users request a withdrawal to L1 ?

Haven't been able to find the answer elsewhere, so thanks a lot to anyone enlightening me on this.