If you've already culled the whole scorched earth map a few times (that includes clearing out the wyvern scar), then they're likely already populated on the map. Heatwaves are random, and can even occur twice in one game day. In almost all cases though it would only occur once in a given game day and typically won't occur for several game days in a row. When I was taming one of my better phoenix I had to wait 4 hours between heatwaves at one point.

Here's a quick and dirty representation of where I've found most of the phoenix when I was hunting them (and yes the inclusion of the dunes in the far west is accurate).
Name: Screenshot_268.png Views: 1028 Size: 1.26 MB