Time of year
The 3 Peaks is always best undertaken during the spring or summer as there are longer days and (hopefully) better weather! Any time between June and September is typical.
However, we would recommend avoiding and school or bank holidays as not only do the roads get busy, the parking around Fort William could get very congested. (Every little helps in sneaking under that 24 hours!)
Time on each mountain
The 24hr time for the National 3 Peaks takes into account both walking and travelling. Typically, this breaks down as around 13 hours walking and 11 hours driving.
Many factors can affect both of these elements, but the area most in your control is your speed on the mountains.
We would recommend you factor in:
Five hours for Ben Nevis
Four hours for Scafell Pike (from Wasdale Head)
Four hours for Snowdon
How to do it
First of all, we highly (highly) recommend you get a dedicated driver. Beyond that there are typically three ways to take on the challenge.
1) Maximise daylight (Start Ben Nevis at 5pm and finish at 10pm to allow for more light on other routes)
2) Avoid traffic (Start Ben Nevis at midday, finish at 5pm for clearer motorways going south)
3) 'The Long Day' (start Ben Nevis at 6am and cover Snowdon in the dark, as its path is easiest).
For more information on these options we highly recommend visiting the Three Peaks Challenge website.