Online self-guided single-session interventions (SSIs), which provide a complete mental health intervention in one brief experience, promise to increase global access to evidence-based support. One way to expand current SSIs' reach is to shorten them, but doing so could also compromise their effectiveness. We conducted two randomized trials to test if shortening evidence-based SSIs reduces their efficacy among adult online workers facing mental health struggles. In study 1 ( = 262), the 8-minute "Overcoming Loneliness" SSI reduced loneliness over eight weeks more than a 23-minute version of it ( = 2.64; = 0.22; 95% CI 0.02, 0.41; = .03). In study 2 ( = 1,145), 15-minute, 9-minute, 5-minute, and 3-minute versions of the "Action Brings Change" SSI did not significantly differ in how much they affected depression eight weeks later (s > .14). Our results suggest that longer digital SSIs are not necessarily more helpful than shorter ones.