The guideline I've seen for nimh batteries is that they charge at about 67% efficiency. This means that you need to put in about 1/3 more power than their capacity. So, if the pack was completely dead, you would need to add around 2600 mah to fully charge it. Divide this by the rate of your charger to get the time.

Having said this, it's obvious that the battery would not be fully dead when you begin charging it, so you will need less time. If you keep the charging current low, below the 10% capacity of the battery level, then you minimize the risk of damage from charging too fast or over charging due to leaving the charger on too long.

When I made the switch from standard battery packs to larger capacity and started using some 5 cell packs for certain planes and boats, I decided it was time to get a better charger too. I found there are lots of good chargers out there. I ended up with the [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXEYD4&P=ML]Hobbico Accucycle Elite[/link]. The reason I chose it was because it comes with a 110 volt power supply but can also work off of a 12 volt source, can charge/discharge 2 different batteries at the same time, and includes a digital display to report the status/results of the charge/discharge. I use the discharge capability to occasionally test the capacity of the battery.