What Is the timer period of a timer of a Pic
Unless otherwise configured the timer will use the internal clock the rest of the PIC runs off of to increment. The timer will increment to its max value, although there are MCUs that are capable of incrementing to a particular value, or decrement to zero. I haven't used PICs too often but I think for at least the older ones if you to decrease the amount of time it takes to overflow and trigger an interrupt you would initiate the timer register at a higher value and also do that every interrupt to ensure it keeps overflowing at the specified time.

It should work the same way for a 16 bit timer, although I would look up atomic operations, writing a 16 bit register is really just writing to two 8 bit registers and it takes more clock cycles, you want to take steps to ensure that the PIC doesn't try to do anything while it reads or writes to the 16 bit registers. (ie disable interrupts during multi-register read/writes)

It isn't an exaggeration when I say you really gotta worship that datasheet. It'll tell you everything you need to know.