What was the life expectancy for a WW1 soldier in the trenches?

It completely depended on whereabouts you were on the western front, the time of year and weather conditions, whether you were in the front line or in the reserve or supply trenches and whether or not an attack was taking place. Some men were killed in a short space of time, others served for the duration of the war. One of the last surviving British 'tommies' Arthur Barraclough, who died in 2004 went 'over the top' several times and never got a scratch. Many men were very superstitious believing if they did certain things in a set order then they would survive. Remember, life in the trenches could be incredibly boring if an attack was not taking place with very little happening, this could go on for weeks before you finally went over the top.