The maximum allowable Cartridge Over-All Length for the 9mm is 1.169". Some reloading manuals list the minimum Over-All Length for different loads, e.g., 1.100" for a 124 grain Jacketed Bullet when using Accurate #2 powder, according to my Lee manual. I would think that you could safely load the cartridge longer than that, up to the 1.169" maximum.
Is there an optimal length of a 9mm cartridge for reliable feeding in semi-autos? Does it vary with brand/type of pistol?