Specifically thinking of the 18th and 19th century (7 years war through ACW) through earlier would also be great. These battles are stylized in art and entertainment as men standing shoulder to shoulder on an open field. While I understand the importance of massed fire and inaccuracy of the weapons (especially during the earlier periods) and the reliance on the bayonet charge... I can't help but think I have a disconnect somewhere.
If those were the tactics, how long would a specific regiment last "engaged" ? I can't imagine soldiers would stand for hours getting bombarded. I know battles may last for days, but what about actual units on the open field firing?
One particular example of two lines that stood blazing away at each other for ages that stands out to me is the attack of the French Middle Guard at Waterloo: opposing the advance of the 1er/3e Chasseurs and the 4e Grenadiers was General Maitland's Guards Brigade, which faced them in line but did not launch a bayonet charge at close range as the French advanced. Consequently, unlike many other battalions of the Guard, the Chasseurs and Grenadiers were able to deploy from column into line, despite being under volleys from the Guards Brigade all that time, and began their own exchange of fire with the Guards, which lasted some time (and indeed, only ended when the realised that the other French regiments on their flanks had retreated). I can't find a figure for how long the lines stood firing at each other, but from a starting strength of 1,080, Maitland's two battalions recorded 492 killed and wounded, or nearly 45% losses.
Source:
Paddy Griffith, French Napoleonic Infantry Tactics, 1792-1815