I use the paid version and it doesn't hold the interval exactly. For example, i set an interval of 20 seconds. The application works correctly for about 10-15 minutes, then the interval between shots lengthens and becomes arbitrary: 1 minute, 10 minutes, 6 minutes, etc.
Sony Ericsson Xperia Pro (MK16i), Android 4.3.1 (Cyanogenmod 10.2)