Subject to any variation in your company CAA approved FTL scheme, the 12 hours should count in full if the SBY was taken during the day (presuming your working for a UK operator ?).
CAP 371, Section 22, Para. 22.3 refers;
"22.3 Calculation of Cumulative Duty Hours (all aircraft)
Duty hours shall be added to cumulative totals in accordance with the following:
a) To count in full:
i) Duty periods and flying duty periods, plus subsequent post-flight duties
ii) All standby duty, except that specified in b) i) and ii) below
iii) The time spent on positioning.
b) To count as half the time on duty:
i) The standby duty, when the period of notice given to the crew member by the
operator before reporting for duty, is treble or more than the specified minimum
report time.
ii) The standby duty when undertaken at home, or in suitable accommodation
provided by the operator, takes place during the period 2200 to 0800 hours, and
the crew member can take undisturbed rest and is not called out for duty."