I use APC in my data centre. The batteries will usually last 3-5 years. If you use the monitoring software, it will remind you at the 3 year mark to replace.
When the battery is gone, YES!!!! they will start beeping non stop, and the light in the front should change indicating a bad battery.
I keep spare batteries on hand, I buy mine for about $50 each right from APC, you can order online and they come in about 1 week. The unit will still run fine, it is just that if power goes out, OH WELL.... make sure you don't have anything vital going when that happens.