Creating an effective retention policy for InfluxDB can be challenging, especially when dealing with time series data that accumulates rapidly. Understanding what a retention policy is and how it affects data storage duration, replication factor, and shard group duration is crucial. When setting up a retention policy, considering the length of use case requirements and designating an optimal shard group duration is essential to maximize performance. The ideal shard group duration should be twice the longest typical query's time range, with each shard group containing at least 100,000 points per group and 1,000 points per series. Tweaking these settings may require trial and error, but finding the right balance can significantly save time in the long run.