Download the Datasheet for the PIC that you're using. Read the Timer sections, they explain in detail how the timers work. Your timer tick will depend on the oscillator frequency (Xtal) that you've set for your PIC.

At 4MHz, the timers tick every 1uS. The smallest tick therefore is 1uS. The longest tick depends on a number of factors... Timer0 is an 8-bit Timer, giving a maximum of 255uS (@ 4MHz), Timer1 and Timer2 (if you have them) are 16-bit timers. Interrupts and Flags get set when the Timers roll from $FF ($FFFF) back to zero. By presetting the Timer Registers you can have them count up from whatever preset value you want. Additionally, pre and post scalers are available (depending on your PIC) to extend the range of the timers. Again consult your Datasheet.

The BEST manuals (other than MEL's PBP one) are actually the manufacturers product Datasheets for what you're using. I can't stress enough that you should study them. It's pretty much all that I use, and what every professional engineer uses.

Melanie