People studying an additional language in adult life do so through a diverse mix of self-directed and institutionally-situated efforts, extending over many years; and online and distance mode language learning offer increasingly flexible opportunities for formal study. Little is known, however, of the ways that long-term learners combine concurrent or sequential episodes of formal, informal and non-formal learning over time. This paper takes as its focus the temporal dimensions of long-term engagement with learning an additional language. Drawing on a 5-year case study of learners of Chinese, it defines and investigates the concept of individuals' long-term language-learning trajectories. Time, viewed from an ecological perspective, is both a context for learning development and an influence thereon. Cross-case analysis of language-learning trajectories reveals recurring temporal patterns and sequences of modes and contexts of learning. Formal study in long-term learning trajectories is shown to be episodic. Through-case analysis demonstrates fluctuation and interrelation between different modes of engaging with the language over the course of an individual's long-term trajectory. "Fallow periods" are also identified, in which there is minimal learning activity but continuing affective orientation towards a learner identity. The implications for pedagogy and research of foregrounding temporality in long-term language-learning trajectories are discussed.