Spontaneous writing observed in chats, instant messengers, and social media has become established as productive modes of communication and discourse genres. However, they remain understudied from the perspective of writing process research. In this paper, we present an empirical study wherein keystrokes made by chat users in a game were recorded. The distributions of the inter-key intervals were analyzed and fitted with ex-Gaussian distribution equation, and an argument for psycholinguistic interpretation of the distribution parameters is presented. This analysis leads to establishing a threshold of 500 ms for the identification of pauses in spontaneous writing. Furthermore, we demonstrate that pauses longer than 1.2 s may correspond to higher-level linguistic processing beyond a single propositional expression (functional element of the discourse).