Haste Rating speeds up the casting time of your Steady Shot. Steady Shot is on the Global Cooldown which is 1.5 seconds. You cannot cast Steady Shot faster than this.
If you are a BM hunter with 5/5 Serpent's Swiftness you don't need to do anything. You are already casting Steady Shot in 1.5 seconds.
If you are a MM or SM hunter then you need a 523 Haste Rating to bring your Steady Shot time down from 1.74 to 1.5.