A person convicted of murder in the first degree - which is a class A-I violent felony- as a first offense will be sentenced to a determinate sentence between 8 to 20 years. See: PL 70.70(2)(b).
I'm not sure there is an "average" sentence for murder in the first degree, however, someone convicted of murder in the first degree faces a sentence of life imprisonment without the possibility of parole in New York (CPL 60.06). The minimum sentence may not be less than 20 years. In fact, the statute provides for an authorized sentence of death, but New York's current death penalty statute has been declared unconstitutional.
i hope this helps answer your question. Best regards, Tom Kenniff