Confusion about SPF in makeup

I remember reading somewhere long ago, that once you applied sunscreen, let's say SPF 15, you only ever received that maximum during the day once.  So if you figure that your spf 15 will last you until 1PM, then if you reapplied at 1pm, you would not receive any benefit from it.  You don't start out at zero again every time you reapply.  So, my question is, if you apply your makeup with spf in it at 6am, will its effects run out by the time you actually see the sun?  Isn't it better to wait until when you are actually going to be in the sun and apply an spf 55 or whatever you can find?  And then, be careful, because your effect is only going to last a finite amount of time?  Or, does the spf in makeup not get activated until the sun actually touches it?

Thanks very much