I've heard quite the opposite. Probably depends on what airplane your getting back "in the groove" on. I've spoken to a few FSI instructors lately who've trained guys that have been parked for years and the first few sim's are a cluster F, but it comes back pretty quick if you have a fair amount of time. I have 8k hours and have never had a problem with training events. Everyone is different. If I was being trained on a 747 100-200 again after being grounded for 4 years, then it might be another story.