Some Ni-MH batteries last years without totally draining themselves. However, I have an identical set which were once over-charged on a fast charger and are now absolutely terrible. They hold a full charge, but only for about 48 hours
Generally speaking, rechargeables wear out. Old ones hold less charge for shorter periods. The worst thing you can do is deep-discharge or badly over-charge.
My cordless phones came with 500mAHr nicads which lasted maybe 10 years, it's hard to tell because they were second hand. I've replaced them with various old cells which don't hold their charge for more than a few weeks, but work fine when they're kept on a constant maintenance charge. I now get several hours talk time from cells that were useless in a torch because they were always flat when I came to use them... However! the charging circuit was designed for nicads, and will not charge a fully discharged set of Ni-MH cells because their resistance is too low. They also charge at a lower current than the new cells are designed for, but it's still enough to maintain or top up the charge unless you use them for hours every day.
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