Foreign language professionals invest considerable time and effort in acquiring foreign language skills. Of key interest is how these skills change over time, and which sustainment, or language training activities, are efficacious in maintaining or improving proficiency. This paper discusses the results of mining more than 800 test/re-test records of foreign language professionals. Analyses investigated the extent to which lag time between test occasions and formal language training impacted changes in listening and reading proficiency ratings. Results indicate that certain factors, such as initial proficiency level, affect both patterns of change and the rate at which foreign language skills manifest evidence of loss.