The rule of thumb for new potatoes is 12 weeks.
Do find when they start flowering is always about that time anyway.
Just have a feel for the top 4-5 inches layer of soil around the plants and see what size they are.
I have been having loads of Charlotte and other variety new potatoes for a week or so now, that was exactly 12 weeks from planting.
Do not leave them too long, as they will grow large and their skins become tougher.