I'm useless at working stuff like this out but.
I have a character on test.
Stripped naked - so no regen on items
elf - so no JoaT
focus and med both set down and 0.
I set the peace song 'resilience' running.
that gives a mana regen (among other things) of 2. Watching the figures mana went up 2 then down 5 till she ran out.
I think that's 5 mana every 3 seconds. With decent med/focus/MR I think you could sustain it pretty well.