How long will a DS1307 RTC run on a CR2032 coin cell battery?
There are few factors:

1) As it was mentioned - battery self-discharge. I've checked few datasheets and it's 1-2%/year at 20-23C. If your device will stay in hot place (70C under direct sunlight for example) - it will self-discharge much much faster. Also, if device will be frozen accidentally - battery might be damaged and loose capacity. 

2) PCB leakage - can reach hundreds of nA in case of severe surface contamination in wet conditions. If you manufacture PCB in good place, clean &amp; completely dry it, and then cover it all (including chips) with some protective layer (polyurhetane or something) you can virtually guarantee no leakage through device life. 

So, if leakage won't be an issue, no extreme thermal conditions - device will likely work for about 20-30 years.