depeds on what you consider ok......in a perfect word....If it took you 15 hours to charge a 600 mAh battery with your wall wart charger......you step up tp a 1200 mAh battery then it will take 30 hours to charge using the same wall wart charger.......
mAh is just a term to give us an idea of a batteries capacity......so if you hooked up a 600 mA load to a 600 mAh battery. the battery would last an hour ideally.
You can go bigger if you like with longer charge times....but remember when you're battery was new you were ok........Try hobby city,,,cheap with about a 10 day lag time to get your order