It usually takes at least 3-4 weeks for a dose increase to begin taking effect and it will take even longer to get the maximum response. Antidepressants work indirectly by stimulating the growth of new brain cells. The cells provide the therapeutic response, not the meds, and they take time to bud, grow and mature. Unfortunately, there is no easy way of speeding up the process. If the promethazine isn't adequately controlling your daytime anxiety then ask your GP to either up the dose or prescribe something else.