Rendered fat - does it last forever? (Storage)

I have some jars of rendered fat in my refrigerator from various years:
* store-bought duck fat used for confit, strained, stored back in fridge (2010/2011)
* rendered duck fat (2012)
* rendered beef fat (1/1/2013)
* making rendered chicken fat today (4/29/2013)

Question is - how long does this stuff last? My searches have told me anywhere from a few weeks to a year, but my duck fat from 2010/2011 still looks good and smells fine... too scared to taste it. I hear about fat going "rancid" - is this an obvious smell?

Also, my beef fat has a thin layer of gunk on the bottom (cheesecloth didn't do a 100% job) - does this affect storage time?

Please share your experiences! I have a LOT of beef fat, and if it's going to go bad soon, I'm going to start gifting fat to people!

Thanks.