This took about 6 hours from beginning to end, but next bench should take about 4-5 hours. Most of the time was spent on sanding the wood and waiting for the stain to dry.
Main lesson learned: Drill pilot holes throughout. I used no-drill construction screws, but it didn't pull the wood tight enough against each other. Also, stain all the wood pieces before assembling (as opposed to just the top slats as suggested in the plan). Remember that the leg pieces will not need stain on the flat surfaces, just the sides that will be exposed. Use a square tool to stay as close to 90 degrees as possible.