Battery life depends a lot on how you use the phone, so everyones batttery life will be different, even with the same phone. Look at the battery capacity of a phone to get some idea of the battery life. A good site to see the specs of phone is
http://www.gsmarena.com and they have a compare tool so you can compare phones.
http://www.gsmarena.com/compare.php3
6300 has poor battery life. I stuck a bigger battery into it, but its still not great. Known issue with it, all over the web. Did you do any research before you bought it?
Personally I find the Sony Ericsson's I've owned to have excellent battery life. I have a 2yr old w810i and it still lasts about 3 to 4 days. If you look at the specs of their phones they generally have good battey life. The Nokia 3120 Classic is a nice low end phone. But the screen blank cuts in way too quickly, and theres no way to adjust this. Very annoying.
A phone with good battery life is the Nokia E63. You can see it compared to the 3210 here
http://www.gsmarena.com/compare.php3?idPhone2=2599&idPhone1=2240
Nokia 3120 Classic
Battery Standard battery, Li-Ion 1000 mAh (BL-4U)
Stand-by Up to 290 h (2G) / 288 h (3G)
Talk time Up to 3 h 30 min (2G) / 2 h 45 min (3G)
Nokia E63
Battery Standard battery, Li-Po 1500 mAh (BP-4L)
Stand-by Up to 432 h (2G) / 480 h (3G)
Talk time Up to 11 h (2G) / 4 h 40 min (3G)
Music play Up to 18 h
Nokia 6300
Battery Standard battery, Li-Ion 860 mAh (BL-4C)
Stand-by Up to 348 h
Talk time Up to 3 h 30 min
Pretty clear about battery life there...