Engagement length

You can read my story or not, but the pole is: how long do you think an ideal time is to be engaged before getting married?

So as my boyfriend and I start talking about our future together, I know he is going to propose within the next 6 months (I'm beyond excited!). The problem is, he wants a really long engagement. Originally we had talked about maybe 2yrs or so, but as we talk more and I mentioned 2.5 based on the fact that we want to go to Australia for a year starting this coming fall, he said he'd rather do 3.5yrs. I think that's too long, but I understand his reasoning when he said he'd prefer to be back in the states for a year to settle in first before beginning to plan the wedding. I'm a bit lost as to where to take the discussion next.