If hand disinfectants are claimed to be effective within 15 seconds, e.g. for MNV*, do hands have to be disinfected for only this period?

No.

Background:
Current data state that hand disinfectants can reach their effectiveness within 15 seconds, depending on the pathogen. This shortened period is not sufficient in clinical practice to achieve high-quality hand coverage ¹. Therefore, both the World Health Organization (WHO), the Federal Institute for Drugs and Medical Devices (BfArM) and the Robert Koch Institute (RKI) recommend an exposure time of 30 seconds for hygienic hand disinfection.

*Murine norovirus
References:
¹Kampf, G et al. (2008) Influence of rub-in technique on required application time and hand coverage in hygienic hand disinfection. BMC Infectious Diseases 8:149.