its 5% for every 3 hours.

20 5% in 100%

20 x 3 = 60. So it will be 60 hours for 0 to 100 - but it would not be advisable to allow your players to get anywhere near 0%. I wouldn't be training players who are anywhere near that. In fact I never train players unless they will still be over 50% AFTER they have completed training.

As to training, it depends on the age of the player and also the intensity that you are training them at. With hard it will take more than it would on normal.