The iPad 2 has a 25 watt-hour battery. The third-gen iPad has a 42 watt-hour battery. The following estimates assume a simple calculation of (watt-hours)/(watts) = (hours), which is not necessarily true in the real world (see below).
An iPad wall charger outputs 10 watts (5 volts @ 2 amps). Therefore, it could theoretically charge an iPad 2 in about 2.5 hours, or an iPad 3 in 4.2 hours.
An iPhone wall charger outputs 5 watts (5 volts @ 1 amp). Therefore, it could theoretically charge an iPad 2 in about 5 hours, or an iPad 3 in about 8.4 hours.
A typical USB 2.0 port outputs 2.5 watts (5 volts @ 0.5 amps). Therefore, it could theoretically charge an iPad 2 in about 10 hours, or an iPad 3 in about 16.8 hours. (Some USB ports output as little as 0.5 watts, resulting in theoretical charge times of 50 hours and 84 hours, respectively.)
All that being said, battery charging is not a linear process, so the actual charging times are probably slightly longer than what I calculated above and not necessarily direct multiples of each other, especially once you start calculating partial charge times (i.e., 80% to 100% rather than 0% to 100%). This is just a theoretical estimate to give an idea of the scale between the different charging sources and the new iPad vs. the iPad 2.