Hi,
OK, so assuming a completely linear risetime (which most likely isn't the case but anyway) it'll be 2.125us from 0.5V to 4.75V.
If you run the PIC at 20MHz the timer will have a resolution of 200ns so you'll get a reading of around around 10 for a "good" pulse (provided the window comparator is fast enough). If you get a PIC capable of 32MHz, 40MHz or even 64MHz the resolution will of course increase.

/Henrik.