The imagery is of something being filled until it is full and can't have anything else added to it, like a cup full with water.
So while the jews were living with the concept that the kingdom will come sometime in the future, after MORE time will have passed - Jesus is saying, there have been enough time. Whatever time we will add to this trough waiting, will not bring the kingdom any more than it is now. The cup of time is full, so the kingdom is not in the future but it is now.
One interesting aspect of this is that we would assume there was a period when the time was not fulfilled - like all the time from Adam to Jesus. But that does not have to be true for this saying to meaningful. If the gospel is eternal then after ANY time has passed, the time will be fulfilled. So what he simply is saying, "the time is now, have always been now and will always be now".