How long does it take for a hard clam (Mercenaria mercenaria) to reach sexual maturity?
According to Ebersole 1987, Mercenaria mercenaria reaches sexual maturity at two years of age although some individuals can reach maturity at 1 year of age under certain growing conditions.  The mean size of individuals reaching sexual maturity is 33 mm with males somewhat smaller than females.

Ebersole cited a few studies that estimated fecundity of M. mercenaria. The results were highly variable. Individuals of about 60 mm in size released between 2 million and 6 million eggs.  Other estimates were as high as 25 million eggs.  Ebersole also cited a paper by Bricelj and Malouf (1980) where they found a general association of increasing fecundity with increasing clam size.  I don't have access to that paper but a book called The Great South Bay edited by Schubel et al. 1991 discussed the results of Bricelj and Malouf. Scroll to page 45 of the Google Books preview.  They show the mean fecundity and size as follows:

Seed (&lt;48mm): 1.6 million eggs / individual
Little Necks (48-70mm): 2.9 million
Cherrystones (70-79mm): 5.9 million
Chowders (>79mm): 6.3 million

However, the figure on page 45 of Schubel et al., and taken from the Bricelj and Malout (1980) paper, shows a lot of variability in fecundity at different size classes.  Hopefully, the citations in the Ebersole paper will point you to other useful studies. If you don't have ready access to the book, your local library may be able to get it for you on loan.

Literature Cited

Bricelj, V.M. and R.E. Malout. 1980. Aspects of reproduction of hard clams (Mercenaria mercenaria) in Great South Bay, New York. Proceedings of the National Shellfish Association 70: 216-229.

Ebersole, 1987. Species profiles: Life histories and environmental requirements of coastal fishes and invertebrates (South Atlantic). Hard clams. US Fish and Wildlife Service Biological Report 82 (11.75).

Schubel, J.R. et al. (editors) 1991. The Great South Bay.  State University of New York Press, Albany, New York, USA.