First find a good memory device that can handle as much of data as you want to spent your money on.

Then calculate the sample store period for that device.

In the end, you'll have a low quality sound recording device.

Some tricks to gain memory and still keep your basic functions running:

Use RAM to prevent worn-out operation of a flash or EEPROM device.
Sample your signal and store it into 4 bits instead of 8. Two NIBBLES in one BYTE.
Make a protocol to save memory so you prevent logging noisesignal. Store a 'NOSIGNAL' and 'TIME' into your ram device. Smart VOX logging!
Buy a cheap phonerecorder and save you time, the effort and your money...