If you have brought a nuc of bees, then you'll want to feed them sugar syrup to help them 'draw out' any new frames that you have in the brood box. You can stop feeding when they have drawn them all out and are using them to store either brood or stores. Depending on the season/weather/bees, this could be a week of feeding, or 4 weeks of feeding. Even if there is a strong natural nectar flow going on when you collect them, still offer them food. Although the bees are not that fussy, a 1:1 sugar/water solution is fine.
If you have brought a full colony of bees from us, then you shouldn't need to feed them. They are generally sold big enough and ready to add a super straight away so you can't feed them.