I am a TA in upper level organic Chemistry labs. Usually, these courses are the last set of Chemistry labs Chemistry students take. My lab day is divided into three sessions, prelab session, instruction session, and lab experiment/practical session. Each lab day takes about 5 hours including a 15 to 20 minutes short lecture about the experiment for the day. I typically arrive early to lab time and ensure all the experiment reagents are ready. I also ensure all the safety equipment such as the eye wash stations and safety showers are working effectively. If the lab is in order, I will print out the lab experiment procedures. These are given to each student to guide them through the experiment.
During the instruction session, I will introduce the experiment to the class and give instructions on lab experiment procedures as well as go through information written on the board with the students. This session usually spans through 20 minutes. Students sometimes ask questions for clarification before, during or after the lab experiment. The experiment or practical session is usually the longest and can take about 4 hours. During this session, all students are fully engaged, performing the experiment, while I go round supervising them to ensure they get their experiments properly done and completed for the day.
At the end of the experiment, the students sign off on the attendance sheet after cleaning the working areas, and reaction vessels to be ready for the next lab day. I ensure the lab room is clean in compliance with the safety regulations.