If my plans are firm, I start looking on and off once the plans are firm. I booked 7 months in advance for a hotel in Anchorage for last summer when there was a price on hotwire of $50/night plus taxes/fees. Bid up to and slightly over that on PL (so the total all-in prices were equivalent) and they didn't bite.
Same for this summer - needed one night in Anchorage and snagged a hotwire price of $64 for a similar hotel, confirmed late last year. The nice thing about this win? Hotwire later downgraded this hotel from a 3* to a 2.5 star though at the time of acceptance the star level was almost irrelevant since I pretty much knew which hotel I would get anyway. But they gave me a $25 credit toward a future purchase when the hotel was downgraded.
Moral? Never too early to start but be sure your plans are firm.
John