On a Friday afternoon I would expect lines getting over the border. The ferry will be a huge line. If you don't have a reservation and are pulling a boat, I wouldn't count on getting on the first ferry. You will likely wait 2 to 3 hours. I have learned over the years to avoid the Tswassen-Duke Pt ferry on a Friday going and Sunday coming unless I'm reserved.
I just tried to book a reservation for next Monday and the only thing left was the 8:15pm and 10:45pm boat. So these reservations book fast. The truckers use them almost exclusively. They allocate two lines to reservation on overheight rigs and then after that the boats will hold about 1 and 1/4 more lines give or take a few rigs. Coming back I usually hit the border in the late evening after 10pm and get right through.