Typically, I get a 'feel' for the situation. I look back at my previous turns, the sorts of spells I've flung out, and I decide to myself "Okay. After this spell, I've got one more spell left in me before I need to take a breather/my mana pool's spent."
Sometimes it's an efficient and logical method of putting your character in a potentially hazardous situation, and it gives other players something to do.
One of my spellcasters is a Blue Dragon Whelp who, for all of his knowledge and understanding of the arcane at a higher level, runs dry on mana real quick.
The natural arcane proficiency and pool of your character's race (e.g. Dragons, Ethereals, Demons, Blood Elves) makes a good deciding factor in this, as does the length of time your character has been practicing his or her craft. (A human Archmage is not only going to be able to fling tougher spells, but is going to have a far larger mana pool compared to a human magi apprentice)