8. There are two inlet pipes, A and C, and two outlet pipes, B and D, in a reservoir. To fill the reservoir, it takes 3 hours for pipe A alone and 5 hours for pipe C alone. To drain the reservoir, it takes 4 hours for pipe B alone and 6 hours for pipe D alone. Currently, the reservoir is $\frac{1}{6}$ full. If the pipes are opened in the order of A, B, C, D, and each pipe is opened for one hour per day in this cycle, how long will it take for the water to start overflowing from the reservoir?
8. $20 \frac{3}{4}$ hours
8.【Solution】Four water pipes, A, B, C, and D, are opened in sequence for 1 hour each, totaling 4 hours. The amount of water filled in the pool is: $\frac{1}{3}-\frac{1}{4}+\frac{1}{5}-\frac{1}{6}=\frac{7}{60}$.

Adding the water already in the pool, the total amount of water in the pool is: $\frac{1}{6}+\frac{7}{60}=\frac{17}{60}$.
After another four 4-hour cycles, which is 20 hours later, the amount of water in the pool is: $\frac{17}{60}+\frac{7}{60} \times 4=\frac{45}{60}$. After 20 hours, only $\frac{1}{4}$ more water is needed to fill the pool, i.e., $1-\frac{45}{60}=\frac{1}{4}$. The time required to fill this $\frac{1}{4}$ with pipe A is $\frac{1}{4} \div \frac{1}{3}=\frac{3}{4}$ (hours). Therefore, after opening pipe A for another $\frac{3}{4}$ hour, the water will start to overflow. Thus, $20+\frac{3}{4}=20 \frac{3}{4}$ (hours) later, the water will start to overflow from the pool.