As long, or short, as you want. The potatoes aren't going to grow any larger now so you can dig them as soon as you wish. If you choose you can just dig a root as required but I wouldn't leave them in the ground beyond about the end of September.

Depends on how sluggy your ground is. Here its best to lift asap if you haven't killed off the slugs. Gardeners used to build a clamp for them, pile the potatoes up and cover with straw I believe.

Ah well, we don't have a great problem with slugs. They prefer our precious seedlings and flowering plants.

Ours will eat anything, even pelargoniums!