Not long is the answer, and that is with a decent, branded, alkaline PP3. I now use a pack of 8 NiCd (AA size) in a Maplin battery case (with a handy PP3 clip on the end) and have Gaffa-taped it to my Autocom with the PP3 connector coming from the Autocom and connecting dircetly to the external battery pack. The whole lot fits inside a small camera case with my mp3 player and a lead going to the Sat Nav, and it can be used on any bike. The batteries last for days and can easily be recharged. 8 NiCd produce just over 10 volts when charged, and this has not been a problem with the Autocom so far (5000 miles)
Nick