How to calculate PIC sampling frequency

Hi, I am using PIC18F458 with 10 bit ADC, and 4Mhz Oscillator. Is there anybody knows how to estimate the fast sampling rate for single channel?

thanks

Min

Are you saying,

you will define a sampling rate say 50 microseconds, then, when you get a sample you would like to know how long was the actual sampling, was it less then 50 or more?

Is this your question?

or you may already know that you can define the sampling rate as

DEFINE ADC_SAMPLEUS 50 'ADC sampling time in microseconds

You change 50 to say 25, then you get two times faster sampling rate.

If that was your point.

or you would like to have a different sampling rate for a specific channel?