The time will vary depending on the model of direct to garment printer. Actual print time will be the same for models based on the same Epson engine, but load, unload times will vary based on the proprietary functions of each model as well as the platen system for the machine (some platen systems lend themselves to faster load/unload cycles which makes for better production of multiple garments). Suffice to say that most of teh Epson based small format printers currently being sold will produce a single A4 size print faster than the shirt can be cured, meaning you will need at least two heat presses to realize the true speed of the printer.
Hope this helps.