How long does it take for L1 visa to be approved?

It depends on a couple of factors: 1) workload of the USCIS; 2) the preparation of the petitioning packet and 3) whether the petitioning choose premium service.
Depending on where the company is, the petition is processed in different service center. How soon you can hear back from USCIS depends on how busy the center is. Vermont, for instance, promises to take one month to process L1 visa petition. California, on the other hand, promises 2 months. However, it is uncommon for service center to take longer than they state on USCIS website.
If the preparation is not sufficient, USCIS will reject (not deny at first), which drags the petition long.

If the petitioner choose premium service, the USCIS will guarantee a decision within 15 days.