Pretty terrible. The first data that springs to mind, Geoffrey Parker provides in The Military Revolution, where he catalogues the astounding wastage of men in the Swedish parish of Bygdea. In 1620, there were about 400 men of military age, of whom 36 were drafted to fight in Germany. In 1639, there were still 36 men from the parish in the army. However, there were only 170 of the original 400 men left in the town. Besides them and the thirty six in the army, 200 men had died fighting for the king in Germany. That would be one man dead every 1.1 months for 228 months. If we assume that the newest soldier will only die after every other man who was sent to fight before him, that gives him about forty months purely as a mathematical average.