The short and long answer is as many times as it takes. Your gut reaction in the first draft may seem like the ultimate ending for your character but this is where rereading and editing comes into play. As you read through your first draft try reading it out loud to yourself because will give you a better understanding of the flow. And better the flow the better the lead up to that all important pay off; your characters untimely demise.
While I was writing my stories for The Collection: Flash Fiction & Short Stories I would stand in of a mirror and read the words out loud. And each time I came a death scene I would picture how a reader would react to it. Will the reader see this coming? Will they be shocked, appalled, or devastated? You have to separate yourself as the writer and immerse yourself as a reader.
Most importantly you have to use trial and error to that perfect death. The larger the pool of options the greater the chance you will find the perfect fit. And that perfect fit may be option number one or the last thing you think of.