Pain monitoring and diagnosis are crucial in seeking to improve animal welfare. This pilot study aimed to investigate the impact of long hours observation on pain assessment and the intra-observer reliability in piglets using video recording. A total of ten piglets, five from the control group (sham castration; pain-free) and five from the pain group (surgical castration; pain-state), were video-recorded immediately post-castration. The videos were randomised and assessed by an experienced observer using the Unesp-Botucatu Pig Composite Acute Pain Scale (UPAPS). The same ten videos were watched at three different times (trial initiation, half-way point, trial termination) with a four-week interval between them. During the four-week interval periods, the observer watched an additional 360 videos from another study to simulate long observation periods. For the pain group, no differences were found in the post hoc test for the UPAPS total score, and most of the UPAPS items. In contrast, for the control group, the UPAPS total score was higher at the half-way time-point, and no differences were found between UPAPS items. The intra-class correlation coefficient (ICC) inferred ‘very good’ intra-observer reliability for UPAPS total score in all time-points of assessment for both groups. Video-recorded pain assessment is a reliable method to assess pain in piglets given that observation duration for pain assessment had only minimal impact on the UPAPS total score, and no differences were found among most of the items. From an animal welfare standpoint, video-recorded pain assessment is a non-invasive method, that can be an additional asset for pain research.