I have no idea, actually. But even if he would invest 16 hours/day into the simul, he would have less than 3 minutes for each move! Maybe that gives us somewhat of a chance after all. :)
Im Attila Turzo - Simul

I think if he achieves a 80% overall score, that would be a great performance.
He said somewhere that he was planning to use ~15s./move.

I don't think your math is correct. Besides, why wouldn't he use vacation time?
(3*16*60)/1000=2,88
Of course he's gaining time once players drop out.

It's three days a move, he only has to make 334 moves a day. 16 hours a day, that's nearly three minutes a move
That's what I was saying. I don't think it is necessary for this to hold that we all make our own moves instantly, only that continuously players are making their moves while he is doing his job - such that we eventually have all made our moves within those three days. I find this assumption quite reasonable (well, more or less). However, I didn't take vacation time into account, that's true.

He said somewhere that he was planning to use ~15s./move.
That sounds reasonable - he then would have to set aside about 1 1/2 hours a day for the simul.

Playing these 1000 games simultaneously is going to be a great challenge. I guess that with his time, he may be able to score at a chess.com rating level of around 1850 or so, if he manages to do really well. Something like 700 wins, 50 draws, 250 losses or better looks like a great result to aim for, depending on the exact strength of his opponents.
Being able to do something like that is definitely exceptional; at that level (number of games combined with playing strength), I basically remember only one single precedent:
FM Kacparov (at 13 years of age, FIDE rating ~2300) has been able to play ~1000 chess.com Online Games simultaneously for quite some months, finishing all of them while adding new ones, not all of them at 3 days per move, many of them with strong opponents of ~1800-2000, mostly scoring an average rating of about 1900, surely becoming a true expert on all the fine points of using chess.com in a highly optimized and time-efficient way as well as possible for so many games. He typically may have used 2 or 3 hours per day or so.

FM Kacparov (at 13 years of age, FIDE rating ~2300) has been able to play ~1000 chess.com Online Games simultaneously for quite some months, finishing all of them while adding new ones, not all of them at 3 days per move, many of them with strong opponents of ~1800-2000, mostly scoring an average rating of about 1900, surely becoming a true expert on all the fine points of using chess.com in a highly optimized and time-efficient way as well as possible for so many games. He typically may have used 2 or 3 hours per day or so.
I was 14 and rated about 2200 at that time :) My top number of games was about 1220, not sure about the exact number. I used 1,5-2 hours daily, playing about 10-12 moves per 1 minute.
So anyone who frequents the forum, Would know that "Im Attila Turzo" Is holding a simul, Hopefuly against 1000 players "1000 Bright Minds" What are your predictions.
My prediction
W: 900
L: 13
D: 87
As for my game with Im Attila Turzo hopefuly i can last 20 moves :)