I'm not a healer, [caveat, talk to your doctor] but my understanding is that to a point, dose correlates to duration. That is, if you take more, the effects last longer. You certainly shouldn't need to drink the whole flask at once, assuming it works for you as I expect.

That said, I might need to direct you to another source for full information on ... Taking a flask's worth at once might be unpleasant, but shouldn't do you long-term ill. [He's heard of Templars forcing than much on a mage at a time, though that observation feels better unspoken.] An even larger dose might have ill effects; I'm not sure. Enchanter Isaac would be a likely source of information, though you may need to explain your condition to him, and I can understand that prospect giving you some pause.