I dont train them if they are below 80%, usually. It depends though when my next game is.
If there are 15 hours to my next game that is 5 timeslots of 3 hours each or 25% condition, so I will put all my guys down to ~74% so they are 99% just before the game. Usually I do this twice a day so I never waste a free condition increase. The most frequent for me is the 4 timeslots or 20% which is perfect since I only have to go down to ~79% and it does save on the injuries (I hope), because if it doesnt then there is something seriously wrong with the injury engine.