As Theo notes it all depends. In traditional Mendelian genetics in plants it did not take long. Because most changes in nature (natural settings) are slow, genetic change often matches that rate ( which can be little or none). However when natural changes are fast we see extinction (dinosaurs) or quick adaptation (Theo's moth). Where man intervenes with high fish densities and stunting in ponds over several generations I would be cautious. That might start the selection process in favor of small fast maturing BG.

Bob very interesting. You are describing Bruce's work on CSBG. Private hatcheries have been doing that for years. You would be surprised at the newness of genetics in Fisheries. Genetic testing is in its infancy wrt fish. The question often becomes what is better. Larger , faster , more prolific , color change adaptation , cold tolerant , there are so many variables. By selecting for size or growth in one situation may not give a better fish . Some of the geneticists are providing a warning in their writings about fish that we (assuming the new genetically improved fish get into the normal population) are taking unknown risks with nature. For example stocking fish outside their native populations (including Fla LMB). Until we know a lot about the total BG genome we don't know all the changes that are occurring. What if selecting for large size reduces fitness for avoiding disease or color change ? Have we then made a better fish ? BTW I support what Bruce is doing and your though just pointing out we don't know very much and should think about possible consequences. The reason we have not seen more work is the costs are high to do genetic research and the field is new and there is little money to be made in fish. People first and things people require (the human genome and food plants and animals first).