To try and keep it alive, I'd offer it a good portion of Isochrysis galbana (T-Iso), Chaetoceros sp., and/or Thalassiosira sp. phytoplankton daily.
For how long it will take to grow: that depends on what species it is. I don't know if it's a clam, but it is definitely some kind of bivalve (bivalves include clams, mussels, oysters, scallops, etc.). Some bivalves (like the giant clams) get absolutely enormous and would probably tennis ball size pretty quickly. Other bivalves, however, stay tiny forever. So, without knowing exactly what species you have, it's impossible to say how long it will to take grow (or if it even will).