webinars will be time sensitive while whitepapers are not. I tend to keep to a 7 day window between emails (Europe 14 days). Once they take action I move them to a 3 day stream. after 2 failed offers they go back to 7 days until there is a lead status change and or opportuinty created.
With webinars I find that my audence is fine with an invite first and a non-responder 7 days later. If closer to the webinar date and the numbers look low, I send a Last-Chance email.