Canon MACRO RING LITE MR-14EX II Mode D'emploi page 14

Cautions for firing continuous flashes
To avoid degrading and damaging the flash unit due to overheating,
do not fire more than 20 continuous flashes. After 20 continuous
flashes, allow a rest time of at least 10 min.
If you fire 20 continuous flashes, and then fire the flash again
repeatedly in short intervals, the safety function may activate and
restrict flash firing. While flash firing is restricted, the firing interval is
automatically set to a time between approx. 8 and 15 sec. If this
happens, allow a rest time of at least 10 min.
For details, see "Flash Firing Restriction due to Temperature
Increase" on page 69.
12