the object of a soft start for a motor, is to reduce the inrush current to the motor and to reduce mechanical strain on mechanical systems. So the maximum time is the time the motor acceleration on the first step is zero, i.e. the motor has reached its maximum speed on the first step. This time can be shortened but only by allowing the motor to take excessive current. i.e. inrush current on switch on star fed = 50A, after some time it falls to 30A, then if switching the motor to delta the inrush is again 50A, falling to 30 amps. I would think this is the optimum time period. It also depends on how many stop/start cycles the motor is going through as the problem is that the motor will over heat if run at 50 A for more then say, 10 seconds. So if in the morning the motor is switched on and off again at evening time then its OK. if its switching on and off continuously , like a a machine tool, then the motor might slowly overheat and burn out.
Frank