On the PICs that I've used, there are registers in the PIC that you can program to tell the A/D on how many clock cycles you want the conversion to be done. The longer it takes, the best precision you get. So it is a balance of how quickly you need the measurements to be taken, versus how precise should the measurements be. There should be something like this in your PIC too, if you read the datasheet section for the A/D. If measurement period is not an issue, make it long, it will avoid headscratching in the future about why your measurements are not stable.