Volume of death star: 9.047E14
Volume of ISD: 69,534,240
9.047E14/69,534,240=13,010,856.23
Amount of time it took to build death star: 20 years
13,010856.23/10512000 minutes in 20 years=1.2377
So at the production rate necessary to build the 1st death star they can build approximately 1 star destroyer in 1 minute.
ISD Fleet count: 25000
Minutes per day: 1440
25000/1440=17.36
They built their entire fleet of ISD's in 17 days.