Timer1 is a 16-bit Timer... it will tick once every 1uS at 4MHz.

Given that, it will count to 65535 before generating a Flag/Interrupt as it rolls back to zero. So, in it's native 1:1 prescaler state it will do this every 65.336mS @ 4MHz.

Prescaling 2:1 will double that... 4:1 will quadruple it etc. You can do the math.