depends- S-to-L? L-to-S? Indoors for some of the SPP? Generally, the more you expect performance to shift (an honest assessment!!!), the shorter the splits might be and the higher the athlete’s level, the longer they’d start out- cause there’s less performance difference possible.
So, when there’s lots of room for improvement, you might start 300s as 2 x (5x60). Top athletes might start 2 x (150+150, or 200+100). In any event, by the end of the SPP, you obviously must be able to complete the whole run, so you need to figure what your progression will be to get there.