The largest specimen taken was 109.8 mm (~4.25”) from Tampa Bay in December 1999. Depending on the growth rate, it was estimated                                      to be about 15 months old.
This suggests that it may have settled in September, 1998.
The highest growth rate recorded in an aquaculture situation was 10.6 mm per month in Singapore (Cheong and Chen (1980). Preliminary growth rate in Tampa Bay (Apollo Beach) was estimated to be 7.2 mm per month (Ingrao).  Out in the Bay, preliminary data shows in to be closer to 5.8 mm per month (Benson et al.).  The literature makes mention of a case where a mussel reached 300 mm. This mussel from Tampa Bay was 109.8 mm and was estimated to be about 15 months old suggesting that it may have settled about September 1998.