There's no set time for battery life Roy.  Some last years.  Others - such as yours, last long enough for the [normal] warranty to run out :emotion-6:

The first place to look for a genuine replacement, is Dell's own 'Dell Replacement Spare Parts & Upgrades'.

As you'll see though, the 6-cell battery costs US$136 :emotion-3:

So you might want to shop around, to see if you can find it cheaper elsewhere, before ordering one directly from Dell :emotion-55:

Parts People for instance, are selling the much longer lasting (85Whr vs the 6-cell's 56Whr) 9-cell battery, for 'just' $149.95 - which qualifies for FREE UPS S&H :emotion-5: