The duration between testosterone peak and endogenous release of LH in male dogs is around 50 min, as observed by DePalatis et al., 1978. In case of exogenous administration, factors such as form of LH (purified or synthetic), dose of LH, animal to animal variation in response to exogenous LH and the previous testosterone peak due to endogenous LH rhythms affect the time of testosterone peak. I would recommend you to collect blood samples at frequent intervals (may be 30 min or 1 h) for a duration of at least 4 h. Prior to administering exogenous LH, remember to collect at least two blood samples at similar intervals.