Roku does not generate the subtitles. They are either in an SRT file, or embedded in the video. In both cases, the Roku player has no control over how long they stay on-screen. SRT files contain that info for each line. If they're embedded, there's nothing that can be done. . What channel are you seeing this with? Netflix, Amazon, Roku Media Player? What player are you using (model number, not name, and firmware version, including build number)?
Dan Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.