The length of time depends on the mah rating of the AA batteries you were using. It all comes down to the mah rating for a given type of cell, and how well it is matched to the cells around it.

Regardless, the 5 cell ni-mh pack will outperform the AAs though, not so much because of the run time (though it will be better) but because it give a higher voltage, which allows your servos to react with more strength and speed.

The OFNA pack I linked to is 10 bucks cheaper than the one you pointed out (from Nitrohouse as mentioned above). Since both have the same mah rating, you might as well save yourself the money.

As for chargers, get a PEAK charger. It will give you longer run times, as well as making your batteries last longer before replacement. Your link points to a ni-mh battery, so you will need a charger that supports that type of cell. Unless you have a power supply, or want to connect it to a full size car battery all of the time, I would suggest a unit that is AC/DC. If you have the money, I would say go for a unit like a MRC Super Brain 977 or 989, because these units can charge two different batteries at once, and charge all types of batteries currently used in R/C. These will give you the ability to charge the battery at a faster rate too. But they won't be ten bucks like the one in your post, they run at just under $100.00. You can get cheaper peak chargers, but those are the most useful I have found for nitro R/C.