Genrally speaking, there won't be any potatoes until after flowering. Having said that, I've had potatoe plants that didn't flower (at least I didn't see them) and produced the tubers any way. Dig gently around the base of the plant around 75 days after planting to see what you have. If the plant flowers, which it should under normal conditions, wait for about 2 weeks before harvesting any potatoes. The longer you leave them in the ground the bigger and better they will become...