The "Reactor Duration" plugin offers a mostly accurate representation how long is left before a reactor runs out.

Currently the plugin isn't fully acurate, passing through a ring, and some other blocks where I haven't managed to figure out the hitboxes yet will not 'refresh' the duration, I am working on improving this though.

Due to lack of available information only the final second of the reactor's duration can always be determined since there is exposed data for an animation that car does in the last second.