A Blaze is the goal of each turn, to successfully score on all dice rolled during the turn in order to achieve a Blaze by scoring on all six dice, and collecting bonus points as described on each card.

That depends on the current card.  If permitted, you may turn over another card and continue play,  but you risk losing all points earned so far in the current turn.

Unless the current card prohibits it, as long as you have scored points on the dice rolled so far, you may stop at any time and collect the points earned up to that point.

On Fireproof you continue rolling the dice until you roll no points.  You may collect all points rolled during the turn, regardless of how many Blazes are scored.

There is no time limit for each turn.  The players may choose to either play each game to a certain score (ex: 10,000 points) or for a specific amount of time (ex: 30 minutes).

If you fail to score on at least one die on the first roll of the turn, you lose your turn and you lose 500 points.  So, yes, it is possible to have a negative score if this occurs early in the game.