Here's what I do, which should work for you too. Start with what I call the "standard home game structure":
25/50
50/100
75/150
100/200
150/300
200/400
300/600
400/800
600/1200
800/1600
1000/2000
Now repeat from 150/300 but with another 0, i.e., 1500/3000, 2k/4k etc. I hope you get the picture.
If you want to have a T100 base, start with
100/200
200/400
300/600
...and then just like the above. Once you're at 1500/3000 you start the loop again but with another zero.
Regardless of which you have chosen, you now have a solid structure. The only thing left is to set the blind lengths. For that you need to know the last level. For non-ante events the "20 big blind rule" is quite common, but since you will be playing for so long I think you should use the "30 BB rule", i.e., the tournament should end no later than when there are 30 big blinds in play.
So if you do 100k stacks and there are 25 players, that's 2.5 million in play. At the level 40k/80k there will be about 30 BBs in play, so it should end thereabouts.
How many levels? If T25 is the base, there will be 22 levels from 25/50 to 40k/80k.
If you want to play 7 hours excluding breaks, that means 420 minutes divided by 22, which is 19 minutes per level.
For a T100 base starting at 100/200, there will be 18 levels to 40k/80k. That means 23 minutes per level. (This also shows my point earlier that the deeper the stacks the more of a turbo the end game is).
So really all you need to do is
- Decide the total duration, decide the breaks etc to deduce the total effective playing time: T.
- Estimate the total chips in play (which is way less important than people think! If twice as many show up that just means 2 more levels).
- Use the 20 or 30 BB rule to estimate the last level.
- Count the levels.
- Divide T with the number of levels to get the level time.
- Profit!