From the specifications I could find (last revised in 2014), new mid 2012 macbooks were all rated for about 7hrs of non intensive use with WiFi on (both 77.5Wh and 63.5Wh models). So if your laptop has the standard 77.5Wh battery, you won't see much over that vs models with standard 63.5Wh batteries which would see a roughly 20% gain in capacity.
However, when you factor in the latest power management tweaks in OSX El Capitan, you may see noticeably better battery life even if you had the 77.5Wh stock battery.
