You cannot make the conversion time constant - the best you can is call for an analogRead at a constant rate.
Say every 120uS to be sure it is done, using a timer, or blink without delay:
unsigned long currentMicros;
unsigned long previousMicros;
unsigned long elapsedMicros;
unsigned long duration = 120; // multiple of 4 microseconds
unsigned int analogVal;
void setup(){
Serial.begin(115200);
}
void loop(){
currentMicros = micros();
elapsedMicros = currentMicros - previousMicros;
if (elapsedMicros >= duration){
analogVal = analogRead (A0); // analogRead is a "blocking' call"? Nothing else happens during the conversion time?
// if so, change this to start the analogRead and then respond to the end of conversion interrupt.
Serial.print (analogVal); // likely to spit out too fast and fill serial buffer on PC
// perhaps save 100 values in an array and then do something with them?
} // end time pass check
// do other stuff while waiting for next duration to pass.
} // end loop