check this earlier thread:http://www.audiocircle.com/index.php?topic=78930.msg773226#msg773226
Yes, you should be able to get 4 to 6 years but it's pretty much entirely dependent on the number of cycles you give the battery and the depth to which you discharge it. Examine the graph closely! You're much better off having a battery [X2] that might seem too big for the job but only suffers a light discharge for any given session of listening. Flogging smaller batteries to low levels of charge will shorten the life enormously.
Interpreting the graph a little:
30% discharge [per use] gives 1200 cycles
50% discharge gives approx 430 cycles
100% discharge gives approx 200 cycles
where the "death" of the battery is taken to be a reduction of retention capacity to 60%.
Note that "100%" discharge does not mean the voltage goes down to 0 volts. The difference between 100% charge and 0% charge can be as little as .5V. The range varies with different types of battery but anything below 12V [for one battery] is getting low and anything below 11V can usually be considered 100% discharged.
Another thing that can exacerbate lifespan problems is leaving batteries in a discharged state. They should be charged ASAP after use.