cfm to cool 1000w hps light

how many cfm do i need to cool an air-cooled light and go through about 15 feet of ducting?

An inline fan to suit your size ducting should work fine just to cool a bulb imo

but how powerful of an inline fan do i need?

my 1000w require a 424cfm, anything less will have heat issues, you want to be able to touch the glass and this will do it