Water supply characteristics don't typically change if the supply is consistent. For instance, if the supply is a big lake or big aquifer, then I wouldn't expect much change. If its a river or a combination of differing reservoirs or lakes, then change could be daily.
Water supply can also change seasonally. For example, in the Western parts of the U.S. you can get drops in minerals in the spring as snowmelt in the mountains runs off and river levels rise.
Climate can also play a role. Lots of rain can slightly dilute mineral concentrations in reservoirs or rivers, drought and low water levels can increase them. Hot weather can increase chloramine/chlorine levels since treatment plants have to add a bit more to keep bacterial levels down. In extreme cases, warmer weather can also increase algae, which can give the water off-flavors and aromas.
The moral is that if you live in an area where the water source varies widely, you might want to get your water checked rather than relying on the yearly water report, which just gives high-low ranges and averages. Once you know your water, however, old data is probably still good as long as conditions haven't changed drastically.