it really depends what your using the ipod for though. I can get around five to six hours of use with the screen and wifi on but as soon as you begin to start using the wifi to acces web pages or download that number dramatically drops to around two to three, and then i can completely flatten the battery from full in less than an hour if I'm playing 3d games while downloading apps in the background.
The 40 hours playing music, generally menas the screen is turned off (one large power drain), and there is no need for WiFi (the 2nd large power drain).
You however are using the largest power consumers heavely - the screen and WiFi.
To improve power life, turn off as many "Push" features as you can. With respect to email, change it to either manual checks or "Fetch" with a reasonable delay between checks.
Unless you need the screen brightness on the highest setting, dim it down. I generally get away with half brightness in a lighted room, and all the way dimmed when using my iPod Touch in bed. I only turn brightness up to Max when outside in the sunlight.
WiFi. First not all WiFi is created equal. If your WiFi setup is experiencing a lot of interference from WiFi base stations your neighbors are using, 2.4GHz cordless phones (your or your neighbors), 2.4GHz baby monitors (yours or your neighbors), Microwave ovens cooking all the time, and/or the distance from your iPod Touch to your WiFi base station is such that you are getting a very low signal, then your iPod Touch will need to use more power to get its signal through.
You may want to disable Location Services for apps that where you do not really need them to know where you are, as that involves using your WiFi.
You may want to disable "In App Purchases" via the Parental controls (make sure you do not forget the passcode you enter for Parental controls). Some apps use WiFi to check if there are any new "In App" purchase options they can present to you. That is going to use your WiFi when that app is active at times you may not be expecting it.
And of course any App that uses the WiFi to continuously check that status of something or stream content to you is going to work the WiFi harder than just some idle web surfing.