Would masturbating on a daily basis make a guy last longer or shorter in bed? why? i say longer because the penis should get less sensitive over time.

Neither: While you may notice something for yourself individually, there is no good evidence scientifically that frequency of masturbation has any effect on how long you last during actual intercourse because the stimulation is so different in that situation. For some guys the additional stimulation of kissing, fondling, pheromones etc, can lead to a shorter time from arousal to ejaculation.