How long until valtrex starts preventing outbreak?

As per my doctor I’ve started valtrex at 36 weeks but wondering how long it takes to fully be in your system and protect you from getting an outbreak? Basically I’m asking how soon can I give birth safely?

I should add that I barely ever get outbreaks but what scares me is hearing stories of moms with sores inside the vaginal canal that they weren’t aware of at the time of delivery.