We cleanly installed Android 5.1 V3.3 image and measured the power consumption.
When C2 turns on, it consumes less than 1uA.
When C2 turns off, it consumes around 15uA.
According to CR2032 Panasonic datasheet, its nominal capacity is 225mAh.
http://www.cr2032.co/cms/prodimages/pan ... asheet.pdf
So its backup time could be around 15000 hours (~ two years).
If on/off ratio is 1:1, it can be 3~4 years.
Can you check the initial voltage of the battery and keep measuring it in every couple of hours to know the power drain?