What is the acceptable vanilla:vodka range for homemade vanilla extract?
According to this recipe, the federally mandated minimum is 0.8 oz per cup - the recipe recommends at least 1 oz per cup for anyone who isn't extracting it with industrial equipment, or about 30g of vanilla bean per 250mL of vodka.

There isn't an upper bound given, but while I have seen "double-strength" extracts containing twice the vanilla, I haven't seen anything stronger, so I would say 60g per 250mL alcohol would be a reasonable upper limit.