The eastern provinces of Canada are big, yet intimate. You drive a long way between major destinations, only to find you're overwhelmed with joy by a small wooden church, a fish stand, a rock outcropping . . . and you end up staying longer than you intended. I've seen it happen, time and again.
So I have two pieces of advice. First, leave a bit of flexibility in your itinerary, because the provinces are full of unexpected surprises. You'd hate to leave Lunenburg without jumping onto a whale-watching boat or the Friday before a weekend fisherman's festival, right? And second, allow yourself time for the long drives -- better yet, bring a companion who can share the load. Except when in Prince Edward Island, you'll log more than a few hours on the road to complete these tours.
The range of possible itineraries in eastern Canada is practically endless (you could even do a kayaking itinerary, or a French-towns itinerary, for instance), but I've focused down to just a few of my favorite places. So, while even a month is not enough to see all of the maritime provinces, realistically most folks simply don't have that long.