The problem in giving a "straight" answer is that it is just plain tough, based on all the possible variable.
99% of the commercial pullets hatched, whether a white egg Leghorn, a leghorn derivative, or one of the plethora of brown egg sex links (there are countless models of these) which are spawn of the huge poultry genetics industry, as tailor made to function in an optimal way for the laying/table egg industry. This means quick feathering, fast maturing, great feed conversion, early onset of laying, high production, thrift, and are generally turned over in one laying season or moulted and allowed an additional laying season. Longevity isn't even considered important.
Given the huge number of strains of Leghorns available and the dozens and dozens of sexlink strains available, making generalizations is difficult for me to hazard a decent guess. A heritage Leghorn? Now, that would be something different altogether to consider.