I don't believe it's true that you can only homeschool half the day. The 900 hours listed here as required is pretty standard nationally for a full year of school.

Here's a link to the law:
http://www.hslda.org/laws/analysis/New_York.pdf

Beware of "hearing" about the laws and customs for homeschooling in your area. It's best to get it directly. There are a LOT of misconceptions out there!

Kriston