I have maybe a four millisecond window at most to read this resistance and act on the A/D value. Timing is everything.
As Steve noted, more details are needed...but as a rule of thumb, a 10 bit ADC acquisition takes about 20uS or less to complete (provided you do not violate the input impedance specification) Further, the lower the input impedance, the lower the acquisition time. If you do not need full 10 bit accuracy, you can cut the acquisition short to further save time - see the Microchip MidRange Manual for details

An ADC example using the registers directly (and not the ADCIN command) is here
http://www.melabs.com/resources/samples/pbp/a2d10.bas
(if you use this latter approach, drop the Pause 5 in the loop)