If it were my pond, I would weigh and measure some every year at about the same time of year, for example end of May to mid June and or in September. Then the year to year data will be most comparable. Or if you are really dedicated measure a few every month or whenever you are fishing. Weighting and measuring does take a little extra time and puts a little extra stress on the fish. I would collect your samples from various parts of the pond not primarily from one section of the 2 ac pond.
How many to measure is up to you and the general sizes and numbers that you are catching. Generally the more often you fish the pond the less you will catch the bigger or biggest bass due to them becoming hook smart/shy after they have been exposed to frequent angling episodes. Frequent angling (angling pressure) usually results the average size of bass caught becoming less as the life span increases of older bass. Thus your younger, less, hook shy bass are more likely to bite artificial baits and these are the ones that you catch or sample most often. After a few years you will not catch one of the biggest bass every time or anytime you go fishing. You will not be able to go out and sample all sizes any time you choose. As the pond ages, the bass are not cattle or chickens in a pen to select a certain group for sampling. So IMO your weighing and measuring should be done whenever you are able to catch one or a few of the midsize and larger bass. Thus fish sampling by angling does have its limitations compared to other ways to sample fish for example electroshocking.
The only way to be able to consistently catch by angling all sizes of bass each time you go fishing is to not make your fish hook smart/shy. How to do this is discussed in the archives under the topic of Hook Smart Bass which is about 1/2 way down the thread.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=255372#Post255372 Body condition factor will change for adult bass depending on season. Food availability and body weight can change throughout the year. Once your original stocker bass become old fish and die you will not have a good idea how old the bass is from just measuring the length. Thus measuring the fish's weight lets you know if it is getting enough food for good growth which is indicated by how plump is it is out of spawning season.
