Many other retailers have been hacked since Target and I think it is doubtful that people are going to hold this against them for a long time. Security is going to be an issue going forward for every retailer. No retailer is immune to this type of problem so there is no reason to hold this against Target.
No, I do not believe that Target customers have forgiven the retailer for 2013's massive data hack. This does not mean that customers will no longer go to Target, I just think Target will receive more cash purchases this year in an attempt for people to protect their personal information.
Target is right up there as one of the largest consumer facing corporations in the US, and as such the company has distinct responsibilities to uphold in relation to its customers. Chief among these is protecting the private information of customers who have entrusted it in the hands of the retailer. Given Target's massive breach of this trust, it will be a long time indeed until consumers regain it.
I think that although they may not think of it, their subconscious is still worrying about it. They will not forget it for a long time. Many people were harmed by this and this is the type of harm that people don't forget. I think they will pay cash or shop somewhere else.