So far the only thing that seems to work is to fully charge your phone and it will start showing the usage. But once you re-spring it after a charge it will show --/-- again
If i remember correctly, last year a tweak came out for this issue or some thing very similar. I know for a fact it had to do with usage/standby times. I think it was made specifically for iPad but I could be wrong on that because it was only available via a pirate repo. I never bothered looking into it or even installing it since I have no use for something like that. If I want to know my battery useage stats I'll refrain from installing stuff from Cydia. Pretty easy for me.