Your question is asking about a stage in the cell cycle. The overall cell cycle can be divided into three parts: Interphase, mitosis, and cytokinesis. Some teachers group cytokinesis as a final mitosis step, but it really doesn't matter. Interphase can be described in general as cell growth. It will also duplicate it's DNA during interphase, which is the cell preparing to enter mitosis.
The DNA is still stretched out in chromatin format when interphase switches over to mitosis, so the chromosomes are not yet visible. The first stage of mitosis is prophase, and it takes the most time of the mitosis stages. That's because a lot happens during this time. The spindle starts to form up, the nuclear membrane starts to break down, and the DNA starts to coil up into visible chromosomes. The simple answer to your question is this. Chromosomes become visible during prophase. A more accurate answer says that the chromosomes are visible during late prophase, which is sometimes called prometaphase.