If your plans are firm, it's definitely not too early to try bidding. Just remember that you won't be able to get your money back if you're successful and later need to change your plans.
I've gotten great deals as much as five months in advance, and as little as a day or two. There's really no "magic window" for the best deals. And sometimes an offer that is refused one day will be accepted the next. It just depends on what they have in their inventory at the time you bid.
If you haven't already done so, check out biddingfortravel.com and betterbidding.com. You should be able to get a pretty good idea what hotels are going for from these sites, and also which properties you are likely to win. I almost always get the hotel I expect after reviewing the wins posted there.