Having previously showed that the workers of the ant Myrmica sabuleti have a sense of time, we here tried to define how they perceive the duration of time passing through the loss of their memory of a learned cue. Thanks to five experiments using olfactory and visual cues and performed on four colonies, we showed that a few hours after having learned a cue and having memorized it in a stable way, these ants lose their memory rather abruptly, according to a sigmoid curve. This shows that ants clearly distinguish events that they have recently learned and vividly remembered from those that took place more than a few tens of hours ago and, for the most part, have been forgotten since. The timing of this memory loss shows that the perception of the duration of time in these ants is affected by their degree of activity: they underestimated the duration of the time that passed when their activity was increased. Although the ants seemed to not react to events learned more than a few tens of hours ago, they nevertheless appeared to keep a weak residual trace of them though not locating them precisely in time. Keeping some faint memory of old events could allow the ants to use the information in the future.