The STR genotype of a cell line can evolve over multiple passages. We therefore recommend regular genotyping of your cell lines to monitor any change.
When a new cell line is established or acquired:
Authentication of an early passage (first week of culture)
As a standard quality control measure:
When beginning a new series of experiments
Once every two months
When cell line performance is inconsistent or results are unexpected
To establish a genetic profile for a new or unique cell line. 
 FDA. General Requirements for Laboratory Controls. 21 CFR 211.160 and 21 CFR 610.18.