My policy on feeding a new colony is to feed if the weather turns unsuitable for foraging soon after hiving. As nearly all my colonies start from natural swarms, they come with a fuel supply for 2-3 days but could starve if they cannot forage beyond that.
I feed honey from my own hives as syrup (2 parts honey 1 part water by weight) in a small contact feeder (honey jar with holes punched in the lid). Honey from another beekeeper is a bit risky unless you can be sure that his colonies are foulbrood free. If you are just starting, sugar syrup would be safer.
I stop feeding when there is a nectar flow and the bees can build up their own stores. Once there is a flow, this happens surprisingly quickly if the colony has started from a good swarm (2 kg). Smaller swarms, e.g. casts (secondary swarms or afterswarms), which are usually later in the season when there is less time left to build up before winter may need a lot more feeding.
If you are working with nature, as permaculture does, then new colonies are being started from swarms at a time when the plant phenology in the locality is generally in the bees favour. Of course, they cannot plan for adverse weather conditions
after a swarm has issued, so this is where the beekeeper can intervene to help survival.
The type of contact feeder I use is shown on this page:
http://warre.biobees.com/feeders.htm