For me forced charging on level, 45mph generally happens somewhere between 1/2 and 1/3 State Of Charge. I've had my '05 MT for a couple of years and can't say I've noticed any change in that regard. What is "normal" in a hybrid is pretty hard to pin down since so many factors go into the charging decision matrix.

If the charging behavior changes noticeably under the same circumstances, it might indicate a problem.....or not. I've heard about recalibrations, where the IMA battery is allowed to completely discharge and only then begins a full recharging, but have yet to experience one in my HCH. As I understand it, these Recals can be triggered by diminished battery capacity (or when actual voltage isn't what the onboard "brain" has calculated it should be). I would have the error codes checked at the dealer at my next service and if nothing shows up just assume all is as it should be.