As the older answer says, it should take 6 hours for a Pokemon to appear. I'd also like to be specific and say that the Pokemon disappears 24 hour after honey is put on the tree, and not an additional 24 hours after the initial 6.
What the older answer leaves out is a niche but incredibly impactful bit of information: the clock that updates honey trees doesn't quite work intuitively and this may cause it to take a longer amount of time for a Pokemon to appear. My basic understanding of it is that the timer that keeps track of these things only keeps track of minutes and does not include decimals to keep track of seconds; i.e., the timer ticks down by one every time a minute passes and does not continually tick down each time a second passes. This timer also stops while in battle, but the amount of time you spend in a battle is tracked and then subtracted from the timer, rounded down to the nearest minute (because, as mentioned before, the timer doesn't keep track of seconds) when the battle ends. This means that you could spend a minute and fifty seconds in a battle and after the battle has concluded, you'd lose the extra fifty seconds. This seems like a small change, but gradually, these time losses will add up and amount to a significant increase in time waiting for a Pokemon to appear at a honey tree. Interestingly, this is also a problem in the remakes, BD/SP.
TL;DR: the in-game clock doesn't work how you might expect it to and this can make the wait for a Pokemon to appear at a honey tree significantly longer.
This video has an in-depth and easy to understand explanation of the glitch if you're curious about it.
