the rate at which it samples is set by ADCSRA. with ADCSRA = 0xf5 the sample rate is 38ksps, ADCSRA = 0xf6 gives 19ksps, and ADCSRA = 0xf7 gives 9.5ksps. to go to lower sample rates you will need to setup a timer to trigger the ADC. using delays is not reccomended as it isnt consistent, although that may not matter too much for your application. be sure to uncomment the cli() if you want to use delay().