If the allegation is of recent lewd and lascivious acts, or of relatively recent anal or vaginal penetration, then the police will likely order a medical examination of the child and a forensic analysis. This may include analysis of the child's body, clothes and the location of the alleged incident.
Different types of decorative concrete involve different processes so timing will vary. However you should normally allow between 8 to 20 days for us to undertake and complete your job.
The rest wll come out when you stand up --- whether you do it a minute or an hour. If you want to wait (and can), staying in bed for 10-20 minutes should be more than ample. The best thing you can do is just lay in bed for 30 minutes and let the sperm do their job. Feel free to shower, pee, etc after that. I think it's a myth because when you think about all those people who got pregnant by mistake didn't wait to urinate or do a hand stand!!