25K starting stacks with 1 rebuy until the end of round 12. 16 players. What level do you see this ending at?
The tournament will typically end by L21 (10000/20000 blinds), or about 7 hours plus breaks -- much longer than your desired 6 hour duration. Dropping the blind levels to 15-minutes will make the event run about 5:15 plus breaks, or right at your targeted 6 hour mark (assuming three 15-minute breaks).
Any suggestions to make this better?
First change I would recommend is changing your re-buy structure. I personally think that 4 hours is too long of a period to offer re-buys (especiallly in a 6-7 hour event), unless it's a low-cost charity event where the goal is to encourage re-buys to generate income for the cause. In any case, the value of a re-buy purchased at the end of L12 is only 13BB when starting L13, which is extremely short-stacked and not a very good investment. I'd recommend stopping the re-buys at the point where they are still worth 20BB-30BB, which is enough chips to avoid having to go all-in whenever entering a pot. In the current structure, that would mean ending re-buys after L9, or three hours.
Your current structure has blind increases ranging from 20% to 100% (averaging 36%), which I feel is just too wide of a range for consistency. By dropping the levels with 20% and 100% increases, you can create a structure with a tight range of 33% to 50% increases with a couple of 25% jumps (averaging 40%), which will also have the advantage of running close to 6 hours yet still allowing longer 20-minute blind levels:
16 players
T25000 stacks (250BB)
20-minute blind levels
re-buys end @ L8 (2:40)
est. EOT @ L17 (5:40, or 6:20 with breaks)
lvl sb bb
L1 50 100
L2 75 150
L3 100 200
L4 150 300
10 min. break, remove T25 chips
L5 200 400
L6 300 600
L7 400 800
L8 600 1200
10 min. break, end re-buy period
L9 800 1600
L10 1200 2400
L11 1500 3000
L12 2000 4000
10 min. break, remove T100/T500 chips
L13 3000 6000
L14 4000 8000
L15 6000 12000
L16 8000 16000
10-min. break, remove T1000 chips
L17 10000 20000 *** EOT
L18 15000 30000
L19 20000 40000
L20 30000 60000