This is an issue of both pressure and time .... pressure drop downstream of the PVB and time required to fill the irrigation mains & laterals. Let's assume he is using a Febco PVB, which starts to dump when downstream pressure falls below 5 PSI (i.e. the air inlet poppet requires 5 PSI to seal properly) and will fully open at 1 PSI downstream pressure per specs. The goal here is to maintain downstream pressure above 5 PSI until the pump kicks in. In addition to a slow opening master valve (or as a possible alternative), another way you could potentially do this is to maintain a "leak proof" system by first making sure you aren't getting any low head drainage (i.e. use sprinklers and drip lines with check valves). The more water/pressure you can keep in the irrigation mains and laterals between irrigation events, the less water/pressure and time it will require to bring the downstream lines back up to operating pressure and flow.