USA L1 visa is the visa issued to employees who are being transferred to America. This visa is a temporary visa which is issued to business professionals or highly skilled trained individuals. One can apply to USA L1 visa if and only if a company is transferring him or sponsoring him. Without the consent of the company you are not eligible to apply for this visa. This visa is issued, based on the approval of a L1 petition which is Form I-129. The period which the non immigrant can stay in US is mostly dependent upon the Notice of Action (Form I-797). This inspection of the notice will determine how long the person can stay in the US. The applicant has to provide proper documentation of his business itinerary. If the officer gets convinced that your trip is feasible then your petition is approved. If your petition is approved than you will be given an I-94 which you have to return to the consulate once you come back from the US. If you want to go back to the US then you will have to take a new I-94. This falls under the renewal category. Mostly your stay is dependent on how long your petition is valid. This changes from person to person.
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 an invalid L1 visa you cannot enter the country again with the same visa. Therefore to apply for a new visa is often problem for most of the applicant 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.