Ah, come on, we can't make any jokes, that sucks.....Seriously, the suction of the mud on the machine is what will likely kill any outright, easy, recovery attempt. I've never seen one stuck like that, but I Dad telling me about one he saw years ago. In that case a D8, had backed over a bank and just the rear part of the machine was stuck. In that instance they used two of the other D8's on the site to ever get it to move. On one of the attempts the pull cable broke and he watched it whip around and make a clean cut on about a 6 inch tree, dropping it to the ground. Ultimately he said there was a distinct, unmistakable sound of the suction of the mud releasing when it finally started moving.
Having a machine nearly completely buried like your describing, I'd hate to think what it would take to get it out. In other words if it took 2 machines to get another of the same size out, when just it's tail end was stuck, I can't imagine what it would take machine wise to pull out one completely buried like that. Even if the mud only doubled the percieved weight of the machine, and if you've ever pulled your foot out of the mud you know it will do that, and more, your looking at needing to be able to pull in the range of well over 100,000 lbs. That being the case, I'd think something like 3 or 4 D9 sized machines, and maybe more would be needed to get the thing unstuck.
That said if I had to make a recommendation to someone in that situation it would start with doing exactly what your talking about, and draining the pond. Once drained get an excavator to dig out from around the machine. Doing this will reduce or eleminate the suction from the mud and make getting the machine out alot easier. Even then I'd recommend at least two D8 sized machines to insure you've got enough pulling power.
Once out drain EVERYTHING. As far as how long is too long, there's no real time frame becuase in reality the second it went under it had been under longer than it should have been. That said I have seen pictures of military vehicles under water for 40 plus years being drug out and gotten running without 'alot' of work. At the same time I personally know of an old IH loader that was sunk in a creek, and flooded over the weekend. In that case the oil in the transmission were displaced as the level in the creek rose, and replaced with water and silt. The owner started it and tried using it to assist the machine pulling it out. While no engine damage occured, every clutch in the transmission was wasted. If you had seen the amount of sludge, grit, etc, etc we got out of every part of that machine that we opened it up, from just a weekend under water, you probably wouldn't believe it. That being said, I won't even go into how to clean everything out because regardless of how clean you think you've got it there will always be something left unless you do a complete dissasembly and clean as you go.
Beyond that all I can say is good luck.