Assuming one is employed on a L-1B visa, how long would it take for them to receive a Green Card? And how long before they are able to initiate the Green Card process?
The two have nothing to do with each other. L1 is a nonimmigrant visa. It has nothing to do with the process of getting permanent residency.
To become a permanent resident through an employment-based category, the employer must petition you (there are certain subcategories that can self-petition). You working for an employer on L1, H1b, or any other status does not mean they will ever petition you to immigrate. An employer could petition you as soon as they want, or after as long a period as they want, or never at all.
And if an employer decides to petition you, there is no requirement for you to be in any particular status. You could be in the US in L1, H1b, F1, B2, or any other status, or you could be not in the US at all, when they petition you. The only requirement is that the employer offers to employ you once you become a permanent resident; there is no requirement that you currently work, or have ever worked, for that employer.