There's no set date to use. You have to judge it based on how they act. They're likely to be pretty skittish the first few days, and you can attempt to grab them a few times, but if they keep hiding, then try again in several hours. With the past few pigs I've had, as well as my new ones, I gave them three days before attempting to handle them. It's best to work into holding them though.
The whole point is for them to associate your scent with safety. And, especially if they came from a pet store, they're likely to be skittish around people in general, because pet store employees aren't always the most gentle. They also smell a lot of different people interacting with them everyday, whether its employees, or people wanting to hold them, so they often get nervous around any people, until you've had them for a few days, and they grow used to your scent always being around, rather than a dozen new ones everyday.
Good ways to work into holding them include hand feeding them, putting out a hand, and keeping it 4-5 inches away from them, allowing them to become comfortable with your scent being close, without putting them into panic mode. I'd also advise holding them with a towel the first several times, that way they feel safer, and are able to hide, and you're safer, in case they get nervous and try to bite.
If at any time, your piggies attempt to bite you in a vicious way (not like they mistook your hand for food) then you should put them away, if you have them out, and back off for the rest of the day.