USA L1 visa is issued for the sake of employee transfer to America. If a company wants to send an employee to its American branch then it has to apply for a USA L1 visa. Similarly if the company wants to send a team to its American branch then it has to apply for blanket L1 visa. The validity of the visa is directly linked to the petition applied. The immigrant can stay in America as long as his petition is valid. Once his petition is expired he has to deport from America on or before the expiration date.
USA L1 visa can be extended if you are in United States. Within USA you can renew your visa and continue staying in America. You need to apply for a new petition I-129. This will only extend your stay but will not give you a new visa. For getting you new visa you need to again apply for another petition in any of the consulates outside USA. Once you leave with invalid L1 visa you cannot enter the country again with the same visa. Therefore to apply for a new visa is often a problem for most of the applicants as they cannot reenter the country. This means they have to leave their adopted house and stay back in their hometown till they get a new visa from the consulate or embassy in their home country.