It varies based on cultivation.

to my knowledge, there is nothing that can be done, but I am no expert.

Just to show what I'm talking about, here's everything again after one day of sun (picture above was this morning; picture below is now at the end of the day). The deep pink flower has faded pretty heavily, especially if you compare to the new one now opening in lower right (I pulled the dead one you see in the 1st shot).

I'm just wondering if anything might be invading the plant that I should know about. Otherwise, everything seems healthy as I mentioned (note the strong branching toward the bottom of the 2nd shot -- this thing is growing like crazy).

This specific varietal (?) is called "Cancun Dreams" out of Gardena in Southern California. The high today (NorCal 9b) was around 90°F and the plant only gets sun until ~2pm.

Thx again!

Depends to a degree upon the variety and upon your heat and sun.

They vary a lot in how much/little they fade, but the fading will be more dramatic in the heat of summer.