I believe you have a total of 10 weeks from you receive a training request until you have to accept, otherwise it will be auto-declined. Also the game is designed to have you training only 1 client per wave, with 2 being "Challenge Mode" and 3 being "Hard Mode" essentially. Also usually the clients have different training lengths, so one strategy would be to take the one with the longest training time and then at about 50% through their training you could pick up the one with the second longest training time and then just before the last offer expires, you accept it. That is how you would have the longest "single-client" time while juggling 3 at the same time, essentially only having to do 2 simultaneous.