“If the pumpkin was healthy when picked and diseases were controlled in the field, the pumpkin can last 8 to 12 weeks,” he said via email. However, he added that jack o’lanterns don’t fare that well – they usually last 5-10 days. The thing is, no matter what you do, no matter how much you try, your pumpkin is going to be mushy after a week. It may last for a few more days, it can last for two weeks in total, but after that, it will rot or collapse.