I'd say about 30 minutes to an hour. As far as how you should relax - you know yourself better than anyone else does. What do you usually do to relax? Is it a glass of wine? Music? Soft lighting? If you're worried about being too tense when she tries the strap on, start slow and have her use fingers first. How well it goes is determined in large part by your state of mind. If you are nervous and worried that it will hurt, you'll tense up and it will. Put it out of your mind, breath slowly and deeply, and focus on how good it feels. If you chill out and use enough lube then it'll be a very positive experience.