The longest level that I have created, is a Level for Lemmix (which I have posted it on the "Lemmix Level Pack Topic") . It is called "The squares fight back (part 2) , it has a time limit of 20 minutes , and it's intended solution, is close to 17 minutes.
The longest levels that I have created for Neolemmix, are "Airborne Rodents" , which has an intended solution, that is close to 13 minutes, and "Cyberlems 2555 - Part 2" , which has an intended solution, that is around 12 minutes. Both levels , can be found in my pack , called "Mikes Lemmings Version 2".
The longest level , that I have solved is a level for Neolemmix by nin10doadict , called "By popular demand" , and my solution for that level , was close to 20 minutes. That level , can be found on the Casualemmings 2.0 pack.