16-24 hours is what I've read for the first charge. Don't discharge it first, just charge it. If you want to understand the reason behind this long slow charge, go to the [link=http://www.rcbatteryclinic.com/]R/C Battery Clinic[/link] and read the article titled "Fast Charge - Is it safe for my packs?".

Edit: the current for most of the wall wart chargers is usually low enough that leaving it on charge a bit too long, for example 20 hours when it only needed 16 hours, isn't going to hurt it. As above, there are articles at the R/C battery clinic that provide more details.