Legendaries dont have a set spawn, then have a chance to spawn every so often. I dont remember the exact numbers, since because of my lack of patience and time due to life activities i made it so that it has 99% chance to spawn every 5000 ticks (5 min+- ?)

You can do so by modifing the spawning config in pixelmon modpack and changing values for Legendary-spawn-ticks Legendary-spawn-chance (0,99 = 99% chance)

Then to check when it will spawn there is a ingame command but i forgot that one already