For a 12 x 12 (a typical LARGE print size for a tshirt) inches print at 1440 x 720 resolution, and with an average print image, it would take less than 10 minutes, and if using ACRORIP V10.1 or newer, that comes with RIPBOOST which speeds up the L1800 by 40%.
For smaller prints such as pocket images, it would only take a couple of minutes.