Haste (2min), barelement(2min), regens (depends on which), refresh(2min=120mp), and buff songs (2min) all have set durations. However some do vary when cast on someone much higher or lower than your own lvl (haste, protect, shell come to mind). So to answer you question there are so stats to change it. However, I think I remember hearing about some top high lvl rdm gear with enhance refresh on it, but nothing solid. There is the nice +regen body armor for whms that increase the amount per tick by +1,2,3 respectively for each regen lvl.
Checked recently on regens: regen I was 1 min 15 sec. Regen 2 was 1 min 30 sec. Regen 3 was 1 min. This results in 125 hp, 360 hp, and 400 hp gained respectively.
If these leaves you wondering what enhancing skill is even for, it affects all your barspells, but not much else.