After defeating a boss and collecting its prize, a brief cutscene will ensue and your health and magic will be restored to full. The health + magic refills occur simultaneously and can lengthen the amount of time that the cutscene takes overall, as shown above.
Each 8 points of magic (1/16th of a full bar) will take 8 frames (0.133 seconds) to refill.
Each heart will take 10 frames (0.166 seconds) to refill.
As a corollary:
A full magic refill will take about as long as 13 hearts.
A half magic refill will take about as long as 6-7 hearts.
The two invincibility items (the cane of byrna and the magic cape) consume magic at different rates:
Byrna uses 16 magic immediately (1/8th of a bar), then after 64 frames, it uses 4 magic every 24 frames thereafter.
The magic cape uses 1 magic every 4 frames.
The magic usage "breakeven point" between the two items is 64 frames, as shown in the graph below:
In most real-world scenarios, however, it's unlikely that you would ever use an invincibility item for less than 64 frames (keeping in mind that you cannot walk after using byrna for ~42 frames and cannot walk after using the cape for ~24 frames, and that a dash takes 30 frames to charge), so byrna is most often going to be strictly better for magic conservation purposes.
Two notable exceptions to this exist:
Negating magic drain while using the cape by performing other actions such as grabbing a wall or slashing repeatedly
Using the cape for brief periods while traveling on a moving somaria platform
Strategies that involve using magic in order to save time may actually end up being slower overall when accounting for cutscene magic refill time. From the above sections, we know that every point of magic use is essentially equivalent to one extra frame in cutscene refill time. Based on the magic usage of various items, we can draw some conclusions about how much time must be saved in order to break even:
Each fire/ice rod shot must save 16 frames to break even
Each somaria block must save 8 frames to break even
Each 4 frames of cape usage must save 1 frame to break even
Using cape for 128 frames (1/4th of magic bar) must save 32 frames
Using cape for 256 frames (1/2 of magic bar must save 64 frames
Byrna:
Using byrna for 160 frames (1/4th of magic bar) must save 32 frames
Using byrna for 352 frames (1/2 of magic bar) must save 64 frames
With half magic, the magic drain is halved, so the breakeven points change accordingly.
(This page is incomplete - check back later for more info!)