I know this sounds silly but... when they start to look thirsty? =P

They both look healthy and assumingly had healthy roots? If so, then there is no critical 'waiting period'. You can wait a few days if you are worried about rot.

Just keep in mind that the roots are not as effective while they re-establish themselves. Start from partial shade and then slowly bring them into brighter spots. Watch for signs of stress / wilting / sunburn and work from there.

If they perk up completely the morning after you water, that's a good sign. If they still look a little dehydrated, that means the roots can't keep up with the sun/heat quite yet.