Sounds like that is just standard li-ion decay time.  Even if you left it outside the guitar for a few weeks and then went to use it you will find that it had lost some if not all of its power.  I have not experimented with mine much as I usually use the VDI, but I have noted that the battery holds up pretty well without use for a few weeks and then it will fade and need recharging.  Not sure of the expected storage life span for a fully charged battery, but if you are using it regularly then you will be keeping it charged and you will probably own a second one as a backup, but if you are not using it very often but then intend to use it, you will need to check it at least 24 hrs before and fully charge it to be sure it will get you through a performance if you need it.