I was probably close to 2 years out on my red stag in Argentina. It wasn't exactly my plan; I had booked about a year out and circumstances changed early enough that I was able to reschedule for the following year without loss, which was nice. So I don't think 2 1/2 years is too soon.
Of course the best price is going to be a last-minute deal. If a client backs out at the last minute and leaves an outfitter in the lurch, the outfitter is going to do his best to recoup the loss or at least mitigate it. The number of people who can leave at a moment's notice and fly to Africa is rather limited. Simple supply and demand will result in a low price. They can be a great deal if it works with your lifestyle and are flexible enough to take what comes up on offer. If my heart were set on a buffalo, I'd spend some time deciding what I want in a buffalo hunt, where that can be found, what my budget supports, where to go (geographically) then finally an outfitter. A good outfitter will be in demand, so a couple years out is not unreasonable. If I didn't really care what I hunt, I'd look through the forum to determine the most common time for a last minute hunt offer, book some time off at that time of year in 2020, get my documents in order, practice shooting my 375 (since I wouldn't know what I'm hunting, I'd practice with my most versatile firearm) and keep my fingers crossed.