GM launches on the day of the keynote/announcement.
Historically,
- 2.0 (iPhone 3): 15 days between final developer beta and GM, 12 to 22 days between each beta
- 3.0 (iPhone 3GS): 33 days between final developer beta and GM, 8 to 14 days between each beta
- 3.2 (first iPad): 12 days between final developer beta and GM, 8 to 14 days between each beta
- 4.0 (iPhone 4): 20 days between final developer beta and GM, 12 to 14 days between each beta
- 4.3 (iPad 2): 30 days between final developer beta and GM, 7 to 13 days between each beta
- 5.0 (iPhone 4S): 34 days between final developer beta and GM, 11 to 18 days between each beta
- 5.1 (3rd generation iPad) 58 days between final developer beta and GM, 14 to 28 days between each beta
If 6.0 (6th generation iPhone) comes out on September 12, it will be 37 days between final developer beta and GM, 14 to 21 days between each beta. If beta 5 comes out on Monday, which is unlikely since it's a US holiday, it will be 28 days after beta 4 and it will leave mere 9 days for GM release on September 12.