Stomach exercises should be included in their routine for those that are able to dedicate time each day to exercising. There are some reports that say that muscles need time to rest, so only exercise a particular group of muscles every other day, while other reports say that stomach muscles are ok to exercise everyday. One way to answer the question on when to do stomach exercises, every day or every other day is first listen to your body. If the stomach muscles are sore, rest them, if not then maybe one day work on them more intensely then the next day, but try to include at least one set of stomach exercises daily.
Besides for appearance sake, there is good reason for wanting to develop strong stomach muscles . This is the core of your body. If it is not strong, the rest of your body will suffer. The good news is that most stomach exercises can be done anywhere with absolutely no machines or gadgets. With a little diligence and some creativity, there are plenty of times throughout the day to sneak in stomach exercises. Your body will thank you for it.