It depends what type. If they are an early season they might well be close. Mid season another couple of weeks and late season give them another month. Make sure they get plenty of water in dry weather. Farmers will be irrigating mid and late season spuds now for yield.
I would dig the soil away very carefully in one of the bags and have a look, put the soil back over if they don't look ready. If they look OK, try taking one and boiling to check taste and for ripeness. They wont spoil sat in the ground for a while longer even if they are ready.