It depends on the how much the flash has been used (P/E cycle used), type of flash, and storage temperature. In MLC and SLC, this can be as low as 3 months and best case can be more than 10 years. The retention is highly dependent on temperature and workload.
|NAND technology||Data Retention @ rated P/E cycle|
|SLC (single-level cell)||6 months|
|eMLC (enterprise multi-level cell)||3 months|
|MLC (multi-level cell)||3 months|