Research on mobile-based assessment systems is still an emerging topic in the mobile learning field. Current research has demonstrated that the use of mobile-based assessment systems seems to have a positive impact on students' learning outcomes and motivation. The paper identifies some factors that influence student engagement with mobile-based formative assessment in a language learning course. Survival analysis of the interaction of N = 86 students from eight English as a Foreign Language (EFL) courses over 5 weeks showed that students with higher levels of self-reported effort and perceived importance engaged for longer periods of time. Perceived ease of use, perceived usefulness and behavioural intention to use were all predictors of the longer use of mobile-based assessment systems. We found that within the first 25-50 min of use, about half of the students might disengage from using a mobile-based assessment application.