A question about Outbreak hunting and respawning Pokemon

When you put up a picnic or change area, it respawns all the Pokemon right? However, once you do, it takes a little while for them all to respawn. Usually the gameplan with outbreak hunting is checking for a shiny, then setting up a picnic and resetting them. But the issue is that you need to wait a little while for them to do so. How long does it take for them ALL to reappear, since I want to wait for them all so I don't miss a shiny. Is there any way to speed up the process?