Several months. Evidence of specialized knowledge is slow to assemble, USCIS processing follows, and requests for evidence are common in this category. Premium processing shortens the adjudication step but not the preparation.
Budget time for a second round
L-1B petitions attract requests for evidence more often than many other employment categories because the specialized-knowledge standard is qualitative. A realistic plan assumes the possibility of one such request and the time to answer it. Sequence the work as follows: confirm the qualifying year and the corporate relationship first, since those are binary and can end the case early; then assemble the technical evidence, which is the long pole; then draft the support letter; then file, with premium processing if the assignment date is fixed.
A Canadian citizen transferee does not need a visa stamp and can seek admission with the approval notice, which saves consular time. Hypothetical example: a Lac-Beauport firm plans a U.S. equipment commissioning for a client in a specific month, files six weeks beforehand, receives a request for evidence, and misses the commissioning window.
Had the file been started four months out, the same request would have been answered with time to spare. Build the assignment date backwards from a filing date that leaves room for a second submission.