Asking how long it took to build a trench is kind of like asking, "How long is a piece of string?" The time it took to construct a trench varied greatly depending on what army we're talking about and when in the war ground was first broken.

A very direct but incomplete answer to your question can be found in the 1911 British Manual of Field Engineering, which expected an infantryman to dig a four-foot-wide section of five-foot-deep trench in four hours. In practice, these expectations were sometimes a little far-fetched. On the Aisne River in 1914, it took days for some British units to dig trenches deep enough for men to stand in without exposing their heads. There were a few reasons for this. First, there were severe shortages of proper shovels and picks at the front lines during the earliest phases of the war. Second, it was simply too dangerous to dig when the enemy was shelling or sniping at you. Thus, most digging had to take place during the night. Lastly, casualties during the mobile and semi-mobile phase of the war in the autumn of 1914 were very high, so there wasn't a ton of manpower to spare. In the British case on the Aisne, only when the Royal Engineers got involved did construction on trenches really speed up. Then we start seeing systems become more elaborate with the addition of communications trenches, cover trenches, and the first dugouts within a matter of weeks, sometimes days.

That being said, in the right conditions, units could lay down a serviceable trench quite quickly. During the encounter phase of the First Battle of Ypres in 1914, when the British and Germans were marching toward one another and before the full weight of the German offensive forced the British onto the defensive, units in the British Expeditionary force dug new lines of trenches every time they halted progress for the day (so, basically, every evening). Here, the British learned their lesson from the Aisne and ensured that front-line troops had better access to tools, and the result was that digging proceeded more quickly. Obviously, these were rudimentary defences, but they would offer sufficient shelter from sniping and errant shelling.

It seems that the second part of your question addresses the issue of how and why some trench systems became so elaborate. For that, u/Georgy_K_Zhukov's excellent answer does a good job of outlining why the Germans, having assumed the strategic defensive on the Western Front, could invest more in their defensive works than the British and French, who remained on the offensive. So I won't repeat what he's already said.

To summarize, given sufficient manpower and enough tools, a unit could construct a serviceable trench over the course of a few hours, but more elaborate defences able to withstand concentrated combined arms offensives took weeks, months, or even years to build out and enhance.

Source: Bodie D. Dykstra, "'To Dig and Burrow Like Rabbits': British Field Fortifications at the Battle of the Aisne, September to October 1914," The Journal of Military History 84, no. 3, 747-773.

IT doesn't address much of your first question, but this older answer may be of interest as it does touch on your second one.