Never unless you are adding fruit, oaking or dry hopping...and many people now adays do THAT in primary as well...
You will find that many of us leave our beers in primary for 3-4 weeks, skip secondary and bottle. Just search for the 10,000 threads under "long Primary" or "no secondary" and you will see all the resaons why we do it, and the explanations behind...There's at least one thread a day on the topic, so it's really not hard to find the discussion pretty much hashed to death.
but if you choose to secondary you should wait til your Hydrometer
tells you fermentation is complete.
Usually on the 7th day you take a hydro reading, and again on the 10th day, if the reading is the same, then you can rack it...
If I do secondary
(which is only when I am adding fruit or oak) I wait 14 days then rack for another 2 weeks...then I bottle.
But that's only if I am dry hopping or adding oak or fruit, whicnh I rarely do, so for me it's a month than bottle,
Honestly you will find your beer will be the best if you ignore the kit instructions, and don't rush it.
But Even Palmer says you should wait with kits...
Originally Posted by How To Brew
Leaving an ale beer in the primary fermentor for a total of 2-3 weeks (instead of just the one week most canned kits recommend), will provide time for the conditioning reactions and improve the beer. This extra time will also let more sediment settle out before bottling, resulting in a clearer beer and easier pouring. And, three weeks in the primary fermentor is usually not enough time for off-flavors to occur.
Your beer will thank you for waiting....