In my mind, when a fork oil change is truly NEEDED, is dependent on how you use your bike and whether there are any noticeable problems. None of the Honda service or owners manuals for bikes I've owned in the last 10+ years have mentioned any schedule for changing fork oil. I have to think that if it were very important or critical, it certainly would have been made a scheduled maintenance item. Ideally, if you were able to do the job yourself, you'd just do it as part of regular maintenance once every year or two since it's a cheap job. Not all of us are able to do this job ourselves however (or have the desire to) for one reason or another.
If I frequently used the ST for aggressive riding in the corners, I'd be sure the fork fluid was changed regularly (what that schedule is, I don't know). Aggressive riding is a case where suspension needs to be in top notch condition. If I used the bike in mostly straight line riding for daily commuting, I wouldn't worry about it until it became obvious that a fork fluid flush was needed, or at "X" miles, whatever that may be based on your own research.
I once had a well known dealer service department tell me that fork oil didn't need to be changed at all (I strongly disagreed with that by the way). I've gone as long as 50k to 60k miles (80k to 96k kilometers) before having the fork oil changed on an ST1100 that I used only for long distance trips. On another ST1100 that I often used to ride aggressively with, I changed the fork oil more often. I would probably have just made it a yearly, routine thing to do but this is one bit of maintenance I never got around to learning so I always left it up to a dealer to do which can get expensive.
So, it looks like no one is able to give you a definitive answer because Honda did not supply one. If you just want to be sure about your fork's health, dduelin's suggestion (post #3) is about as good a guide as any.
