No one can give you exact figures unless they have an x50 and are willing to take the time which would probably involve long periods of time of doing nothing but staring at a PDA, and power settings, volume, brigthenss, etc, will still come into play. Thats why so many review sites just talk about how many hours they got of "normal" use, which includes playing a game or two, watching a video, listening to music and doing some surfing, because it's a real boring pain in the ass otherwise, and it's also just not typical of pda usage either in the abstract or for the individual to just sit there for hours doing nothing but surfing the web over bluetooth. It's just not very illustrative. But one of bluetooths main attractions is a limited power draw. It draws MUCH less juice than wifi. The x50 has an 1100 ma battery which is completely average in capacity, but it does have a fast proc that soaks up juice, though it can be underclocked for power saving. The site i go to said
battery life is average for a Pocket PC, giving about 3 hours and 15 minutes of actual use with the Auto power setting in average conditions consisting of PIM use, working with MS Office documents, surfing the web for an hour using WiFi, gaming for 45 minutes and playing a few short videos.
In practice, your battery life will probably depend a lot more on what you are doing while bluetooth is on and how many other bells and whistles you have going rather than bluetooth itself, especially with a keyboard which isn't going to need to send large amounts of data like a BT internet connection would. you can get a 2200ma battery if your running out of juice too fast, but it will increase the size of the pda, so buy your case accordingly.