One school of thought holds that electronic products will last longer when they are not powered off and back on continuously because this places additional stress on the power supply and electrical components.
Another school holds that continuous power off and on cycles do not affect the life of the electronics at all, and that a product will last longer if it is not used continually.
Good arguments can be made for both schools, so you will have to choose which school that you want to attend.
As far as the Time Capsule is concerned, Apple may have likely provided a clue for you as to which school that they attend. In other words, since there is no "on" or "off" switch on the Time Capsule at all, it is pretty clear that Apple has designed the Time Capsule to be "always on".