Unfortunately, I believe all the pins on the arduinos are only rated for 40mA. So while your power supply might be able to provide more, if you take it from the power pins you should not draw more than that.
Alternative methods of powering are to provide an external regulator (either linear like the on-board one, or switch-mode DC->DC converter) to provide 5V - heatsinking can then be used to deal with highercurrents (or the DC->DC converter option is less wasteful anyway, typically > 80% power efficiency).
Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.
You mean 7v (not 5v) to the barrel jack, correct? Both the Uno and Mega spec sheets now warn about supplying 5v power directly to the 5v pin. It can cause damage.
So, using a 7V input to the barrel jack, what kind of battery capacity is needed considering the unit needs to work for 3-5 (12-hr) days?