Complete turnover can occur in just a few hours, especially in the fall. In the spring it may take longer for the mixing to occur. This is evident when oxygen levels in deep water approach those of the upper layer. Turnover has to do with the breakdown of the thermal barrier that separates the two layers. This can occur in the middle of the summer in high wind. We were curious as to whether wind would be enough to accomplish this and braved a very windy summer day on Pony Express Lake and found the lake had mixed in 3 hours of high wind (40+ mph). Oxygen levels were constant from the surface to the bottom in the deepest part of the lake which indicated the upper and lower layers had mixed. Turnover is a short term event, but stratification can last months in the winter and summer. The EPA article is a good reference.