There's a general rule-of-thumb that anti-depressants take about 2 weeks on average to produce effects that the person can feel. They begin working much sooner but the effects are not felt by the person until the 2 week mark. However, when there is a drug-drug interaction, such as the one occurring with you, it becomes pretty unpredictable as to the timing for any symptoms to be shown.
But something else to add, Epival really isn't an anti-depressant. It's an anti-convulsant but it has been shown over and over that anti-convulsants can and do help with treating some mood disorders, although you may need to pay more attention as there can be a greater variety of side-effects, some of which are very nasty and put you in a place far worse than simply going to the psych ward.
Since you're also in Canada, this may be relevant to you and in addition because of bipolar and mood disorders, it's from the Mood Disorders Association of Manitoba:
http://www.depression.mb.ca/document...EATED-OBAD.pdf
It is a PDF.