My rule of thumb is, if the trainee gets 42 minutes or more, I stick with 1-position training. If less than 42, I go to 2-position. Since 48+ is 48:00 or more, 48 minutes is anywhere from 47:30-47:59. If you just do 1-position, you'd only lose about 1% of the 100%.
As skill that is much higher than related skills take longer to train, but a skill that is lower than related skills trains a bit faster until it gets close to the level of the other skills.