When the timer wraps round it will get loaded with the reload value and continue counting.
If counting up, it is going the wrap around after 16777215 clocks with that reload value. I assume the timers are 32 bits given those values.
What period that is, will depend on the frequency of the timer clock. If you want a shorter period increase both of those #defines (assumes timer counts up)
To calculate an accurate value, you will need to know what the input clock is.
There will almost certainly be some sort of prescaler in there to reduce the main clock. Ideally you want to clock the timer at nice value such as 1Mhz, where a count of 1 will equaly 1us, or maybe 10Mhz for 0.1us resolution.
If generating PWM’s there will be a match register in there also with a value between the reload value and the end of count value.