Generally, the rule of thumb is that your tournament will end when the money put into the pot initially (BB+SB+Antes) equals 6-10% of the
total chips in play.
Since you've instituted a rebuy, you really don't know how many chips will be in play at the end, but you can make a pretty decent guess at it.
In a larger field event (50+), you'll find that there will be at least a dozen guys who play
very aggressively during the rebuy period, trying to accumulate chips. To be on the safe side, you could even assume that
half of your players will be rebuying.
So, if you're expecting 50 people, and playing T5000, that's 250k in play. After 25 rebuys (a conservative estimate), you've got 375k in play. So, you'll probably start heads-up play around the 5k/10k level, and end before you reach 10k/20k.
If you're scheduling your blinds to look like you listed (plus: 1500/3000, 2000/4000, 2500/5000, 3k/6k, 4k/8k, 5k/10k, 7.5k/15k, 10k/20k, and then a 15k/30k level just in case), that's
20 levels. At 20 minutes/level, that's almost seven hours.... for 50 people+25 rebuys. I'd guess closer to six, seeing as how you won't have that many rebuys, and heads-up action usually doesn't take long.
I just saw in your initial post that you're expecting 30 people.
Again, I'd figure for 50% rebuys to be conservative.
A T2500, with 45 buy-ins, will have 112.5k chips in play. Blinds totalling 10% of that is approximately at the 4k/8k level, meaning you'll have 16 levels @ 20 minutes, for about a 5 hour tourney.
A T5000, with 45-buy-ins, will have 225k chips in play. Blinds totalling 10% of that is approximately at the 7.5k/15k level, meaning you'll have 18 levels @ 20 minutes, for about a 6 hour tourney.
Remember, the 6-10% is an
estimate, and will fluxuate depending on your players at the final table. Most tournaments will end much quicker than the 10% level (possibly even a couple blind levels quicker), as amateur players don't really play short-handed very strategically, it's more pushing all-in. Also, remember to add more time for breaks (chip-breakdowns), and allow for a somewhat late start as you'll have trouble wrangling all those players into their proper seats.
Also, with this many players, including a re-buy, the tournament director is going to be very busy throughout the tournament, doing re-buys, shuffling players to balance tables, settling discrepancies, etc. I wouldn't plan on
playing on a tournament with 30 players in it, if I was running it. You'll just end up slowing play down, and rushing yourself constantly. Better to run the tourney more efficiently for the rest of the players, and make sure things go smoothly.
Good Luck!
