I've had my DS for about 9 months now.

When I started, it was able to go for 14-20 hours without a recharge

Now, its down to 4-5 hours before I need to recharge

Is this normal?

I don't have a DS but I know that rechargeable batteries have a life span and tend to wear out depending on use and charging habits. 

Yeah, it's common for rechargeable electronics' batteries to run down after a while. Yours seems to have begun wearing out pretty early, but that could be a result of charging habits, like GG said. Mine's still going strong and I've had mine since the first line of DS's were released.

Good news though, you can buy replacement batteries for relatively cheap off the internet (Amazon, etc.), and they're easy enough to replace.