The time limit used to be 90 days...but then a lot of readers would just fit their predictions to that and say it will happen in 3-4 months. And then people might call back for updates and get strung along for months on the same predictions. That can still happen...but in a weird way, by limiting feedback to a shorter time frame maybe it puts a little more pressure on them to deliver something worthwhile, other than a 3+ month prediction. But they also might feel more impatient to ask for feedback since there's only a short time for them to get it.
The whole feedback system on there sucks and it doesn't really mean a lot since there's no way to rate someone's accuracy over time.