There is no problem with your battery. leJOS 0.8 (and earlier), simply displays the current voltage of the battery based on a full scale reading of 9V. The system menu will tell you the actual voltage. In the next release of leJOS we will detect if a Lego rechargeable battery is in use and if so adjust the full scale reading to be 8.2V so you will get a "full battery" display.
How long they last is very hard to say (it depends on what you are using them for) - for instance if you just turn on the nxt then it will last for a day or so, but if you are running all three motors then it will last much less!
Not sure how long it takes to fully charge (I often just leave mine connected). One of the lights on my batteries certainly does go off from time to time (not sure which one). Maybe you just need to leave it longer...