Your PIC18F2550 has 2k of RAM so I think you should be able to send 512 or maybe 1K of data in a packet. That depends on how you store the data. (8 bit, 16 bit, compressed)

Many compilers have a function 'get ADC value'. This function starts the conversion, then waits for the conversion to be done. If you need to read analog very fast you will need to make your own get analog voltage function. It is faster to process data while the ADC is converting the next data.