It seems like most relevant casts in this game will pulse 1.5 - 2.5 times before the spell goes off. As long as there is any sort of motion in the highlighted circle at all, players who need the game to scream in their face should be satisfied from a visual cue perspective (the difference between a static circle and a circle that strobes is huge, but the difference between a faster and slower strobe is not significant).

However, if SE makes the strobe sync with the spellcast so that it is exactly 1.0 pulse, then it will make the indicator very clean, and also improve timing by allowing players to see more precisely when the spell is done casting.