Timing on positive opk

All my apps have told me I would ovulate on cd14 or 15. This is my first month using an opk and I'm on cd20 (usually 30 day cycle) and got a positive!! How much time do I have to do the BD? I test the opk about 3 times a day and this AM I got the first positive. My husband has a very busy week with work and school this week so I was just wondering what is the window of time we have to get the bd done?