Sort of a general comment. As Sparrow was pointing out, it is usually best for these DC RC or RL charging/discharging circuits to try to understand what is happening rather than to look for an equation to plug into. Of course, if you can't figure it out exactly, plugging into something is a last resort and will give you a better chance.
I was always confused by these things, until I really sat down and started thinking about what it meant by "connected for a really long time" etc and what was happening over time. I actually had to plug in some numbers for time to get it in a couple cases. Of course, even these generalizations can be decieving. A capacitor can be connected across a battery for a really long time and never charge to the full value of the battery if it doesn't have sufficient capacity. But they don't usually throw curve balls like that at you without making it really clear.
Anyway, it may help to google around for a site that you can understand to help you out if you have trouble -
For example -
http://www.ac.wwu.edu/~vawter/PhysicsNet/T...t/RCSeries.html
In my case, for these DC transient circuits, the mathematics (DEs) was no help in understanding them conceptually. First I understood the concepts, then I looked at the mathematics behind them. YMMV.