Note that this is purely speculation:
(Not including completion and airing dates, between days only)
Longest gap from original concept pitch to US airing:
- Growing Up is Hard to Do (1000 Days)
- Fame and Misfortune (911 Days)
- 28 Pranks Later (777 Days)
- Yakity-Sax (666 Days)
- The Beginning of the End (420 Days)
- The End in Friend (420 Days)
Longest gap from completion to US airing:
- Feeling Pinkie Keen (470 Days)
- The Ticket Master (420 Days) (Not too sure about these two, which one was finished first then?)
- The Last Problem (367 Days)
- The Ending of the End Parts 1&2 (364 Days)
- Common Ground (351 Days)
Shortest gap from completion to US airing:
- The Last Roundup (-1 Days)
- Growing Up is Hard to Do (97 Days)
- Look Before You Sleep (99 Days)
- Yakity-Sax (100 Days)
- Non-Compete Clause (101 Days)
6*. Bloom & Gloom (104 Days)
*If you exclude The Last Roundup
I wonder what the completion and pitch dates were. Mathematics was never my strong suit…