You can split the mesh at whatever point in your process you want. Most developers do the splitting as a pre-processing step. For example, at Krome Studios we have an exporter for 3DS Max and Maya that can take a level mesh and split it into cells (the artists call this "gridding" the mesh). The artists specify the size of each cell after trying different sizes and what effect they have on game speed. The frustum culling process in the game can then easily cull cells at render time.