Every month? No, you should change that shampoo immediately . . . two-in-ones don’t clean the hair well (how else would they get the conditioning agents to stay?) so the conditioners tend to cause buildup. The conditioners they use are ones that don’t wash out easily, and the shampoo tends to have a high concentration of them to insure that they don’t entirely rinse away.
But the notion that you ought to change your shampoo frequently is not really true - at least not nowadays. The idea is that using one shampoo will eventually lead to buildup, I think, but it’s not really an accurate one. However, if you DO insist on using a two-in-one, I’d suggest having a regular, non-conditioning shampoo to use once a week or so to make sure you’re not accumulating too much conditioning agent on your hair.