I know of people leaving Valley with about 10 year time served with circa 2000hrs most of which is fast jet. It's easily achievable. Currently you'll need this experience to get a job:
BA - 1500hrs
Monarch - 2000hrs
Virgin - 2500hrs
Easyjet - 500hrs
As for leaving early, you need to do a minimum of 6 years service ONCE YOU HAVE COMPLETED AN OCU. Typically it takes 3-4 years to complete an OCU, so you would be looking at 10 years in before you can leave. That said, it'll be the best 10 years of your life!