In fact, the amount that an infant would receive would be only 5 - 6 % of the weight - adjusted maternal dose. (momtricks.com)
Per Medications and Mother's Milk (Hale 2017, p. 139 - 140) caffeine is in Lactation Risk Category L2 (safer); milk levels are quite low (0.06 - 1.5 % of maternal dose) and usually peak 1 - 2 hours after ingestion. (kellymom.com)
Generally, very little of the maternal dose passes into the milk. (wendywisner.com)