Can you please tell me when is a common cold at its most infectious period. Before it actually 'comes out' or when it is actually 'happening'?

Hi,
The infectious period of the common cold is from about 1 day before you feel the symptoms of the cold, up to about 5 days into the cold itself.
The common cold can last normally about 7 to 10 days.
This information is available, with more, at:
http://www.dh.sa.gov.au/pehs/Youve-got-what/specific-conditions/common-cold.htm