I can't disagree with the general guidelines posted but a lot of people succesfully breed at 2 years. What impact that has on the female - I don't know, and you'll find claims on both sides of the issue.
3 feet and 300 grams sounds like a good rule of thumb though - that very well may mean 3 years for a lot of corns.
As stated - lava is a single gene morph. You need to get corns that preferably express (homo) or at least carry (het) the lava gene.
There's a good book on corn genetics - I don't recall the name of it and I don't own it, I'm hoping to pick up at Sac show in a few weeks. If not there it is online. Someone else can probably give you the name of it - I think it is updated yearly. Anyway - I'd look into getting that book if you want to know what possibilities you have from your corns.
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x.y L. getula californiae (Cal. King)
x.y L. getula nigrita (MBK)
x.y L. getula floridana (Brooksi)
x.y Pantherophis guttatus guttatus (Corn)
0.1 Pituophis catenifer catenifer (Pacific gopher)
0.1 Heterodon nasicus (W Hognose)
x.y.z Elgaria multicarinata multicarinata - (Cal. Alligator Lizard)