I don't now...I recently started a WC and under certian circumstances I can hold my MP at max.
I noticed that if I nearly drain my MP buffing and then start spamming nukes on a KAMA boss.....the MP gains from the passive MP Leech skill will not be able to hold enough mana for me to continue to spam spells.
However, if I'm at full Mana and I'm in an open field with LOTS of mobs.....I can essentially spam my drain with ZERO down time. I don't even lose a quarter of my MP after non stop casting.