Glancing at a catalogue, the fingerlings have a considerably longer days to maturity than do regular, larger potatoes. The general potato rule of allowing some foliage dieback to occur holds true. The only sure way is to poke around in the perimeter of the planting and see if the potato has reached the desired size.

Full maturity is important as the skin will be tougher and the potato will store better. For immediate use, you can get away with earlier harvest.