I purchased some fungicide. The instructions on the back of the bottle say "Apply at 4–6 weekly intervals."
Does that mean it should be applied 4–6 times a week? Or every 4–6 weeks?
It means that the fungicide should be applied every 4–6 weeks, not 4–6 times per week. If it meant 4–6 times a week, it would say "Apply 4–6 times per week". The "interval" between applications is 4, 5, or 6 weeks.