Under 4 days, you've about a 50% chance of passing it to someone else, and it drops off after that.

There's a really well-researched answer at madsci.org, which I'll reproduce here:
The common cold can be caused by a variety of viruses. It is principally caused by rhinovirus and coronavirus infections; but respiratory syncytial virus and parainfluenza viruses may also cause common cold in adults (whereas they cause mainly lower respiratory tract infection, such as bronchiolitis or pneumonia, in children.) As would be expected/experienced by most people, transmission of rhinovirus and coronavirus infection depends on prolonged close contact, and is by droplet spread, or via contaminated nasal secretion carried by the subject's hand/body. The incubation period, i.e. infectious while asymptomatic, ranged from 1to 5 days. And it has been estimated that a susceptible adult has a 50% chance of being infected by 4 days of close exposure to someone with a moderately severe cold. The median duration of the rhinovirus-associated cold episodes was estimated to be around 11 days; with specific nasal IgA antibody response appearring withint about 24 hours. However, this immune response does not become significant until about 1 week after initial infection and only reach a maximal response at 5 weeks. Therefore, although infectivity does decrease over time, it remains moderately contagious in the first week or so; and there is little truth in saying that the common cold is no longer infectious by the time symptoms appear.

I hope this help:-)

Joshua Chai

References:-
1. Ledingham JGG, Warrell DA. Concise Oxford Textbook of Medicine. Oxford
University Press
2. Elliott T, Hastings M, Desselberger U. Lecture Notes on Medical
Microbiology. Blackwell Science
3. Callow KA, Parry HF, Sergeant M, Tyrrell DA. The time course of the
immune response to experimental coronavirus infection of man. Epidemiol
Infect. 1990 Oct;105(2):435-46.
4. Barclay WS, al-Nakib W, Higgins PG, Tyrrell DA. The time course of the
humoral immune response to rhinovirus infection.
Epidemiol Infect. 1989 Dec;103(3):659-69.

The first few days of the cold are when they are most likely to pass it on. That's why its extremely important to wash your hands frequently if you feel like you're getting a cold, and take plenty of vitamin C.

The first few days, you may not even realize you have the cold yet, so these are important things.

It is said that the contagious stage is before the symptoms, but I cannot understand when people are sneezing, coughing and not covering their mouths that they are not contagious?
I think that mucus must contain the virus and any way it rests on the surfaces (like door knobs, keyboards, desk surfaces, etc.) will make it available to 'transmit'. Prevention of this is to suppress coughing and sneezing, to wipe down surfaces that are not longer exposed to the ill individual, and to avoid sharing food with that individual.
The other part of transmitting an infection is the potential new host's mucus membranes be somewhat more easily available for a virus to begin attacking the new body host cells. It would be helpful to humans if the cold virus would become neutral once it hit the air - like HIV supposedly does - but that is not the case with most colds.
By the way, because there are no organs or any conventional systems that look like a living organism, viruses are considered probably not living. So it is not clear what DIES when it stops being effective.
A virus is a holding tank of RNA with trigger injectors on the outside of the virus, waiting to insert the RNA into our live cells. Once that injection has happened, it mutates that cell's DNA, converts the entire cell into a virus factory. This single cell explodes into a bunch of new viruses and the process starts all over again exponentially. Your body will take several days to learn the 'viral code' of the virus and start to recruit your immune system to attack anything with that 'code.' In the meantime, your body starts pushing the virus out of the body fast through a high production of mucus. I am not a doctor, but I believe this is crude layman's description of the process is mostly accurate.

When is a person most infective?
The incubation period for a common cold is usually around two days before symptoms start. People are most infective when the early symptoms of sneezing, runny nose and cough are present.

"Let's assume you're the one with the cold. During the two-week period when you're contagious, research indicates that you'll be most likely to transmit the cold virus when you're experiencing cold symptoms. The period when you're most contagious is when your symptoms are at their worst -- day 2 to day 4 of the cold. You'll become less contagious as the cold symptoms improve." -- http://cold.emedtv.com/common-cold/common-cold-contagious-period.html

Worst case scenario, several days before showing any symptoms up to the day the patient shows no symptoms.

Other than a general timeframe, it stands to reason that if you're in close proximity to people and sneezing violently, that's the time at which someone else is most likely to catch it from you.

You didn't ask, but I'll tell you: the quickest way to stop it at this stage is Zicam, the original swabs. Use them when you're at the point where you can't stop sneezing and often that's the end of the cold.

Sadly, it is before you even know you are sick. That is when you are most contagious. Your best course of action is to stay healthy, hydrated, take your vitamins! :)