That timescale can work for certain campaign types, though. If you're playing a campaign that is centered on a town or village as where the party adventures from(for a length of time anyway), then the witch character could give the evil eye to the farmer than insulted him/her causing his crops to be diseased and die over a span of weeks.
The game doesn't stop or center on the witch character after the evil eye is given, rather it continues on with the party doing what they were, and as they come back and head out over a period of weeks, they see and hear about the effects of the hex. The farmer complaining to some fellows about the unnatural disease hitting his corn crop that he can't fix. He's tried cutting out the infected sections, but it keeps reappearing.
Maybe the farmer connects the dots and maybe not. If the witch keeps causing issues and enough people talk about things, it becomes more likely to find out who the culprit is. A lot of good roleplaying could be done by the entire group over what is happening, including an unscrupulous party being hired to "find out" who did it and making coin off of it.
It's an ability that can work for some types of games, and for the ones that it doesn't work for you don't pick a witch to play.