1 cycle =1000000 / [ 4/FOSC(in MHz)] in µS
FOSC can be internal (inside the PIC) or external , use of a Quartz or an external clock device generator.
for Other PIC18F you can active a PLL multiplicator => FOSC*4 => 1 cycle= 1000000/ FOSC µS
for executime time
Code operation take mainly 1 cycle ( it's explain why we divide FOSC/4)
but some of them can take more than 1 cycle
exemple Goto adresse => 2 cycles
Test true => 2 cycle ,because of jump.
and special math op like Multiply 8x8 take many cycles.