Here are the completed sentences:

1. It was such rotten meat that it had to be thrown away. (SO... THAT)
→ It was so rotten that it had to be thrown away.

2. I spent three hours doing my homework last night. (TOOK)
→ It took me three hours to do my homework last night.

3. She can’t sing as well as she used to. (USED)
→ She used to be able to sing better.

4. The table was so heavy that we couldn’t move it. (TOO)
→ The table was too heavy for us to move it.

5. Laurence hasn’t seen her sister since she left for Japan. (LAST)
→ Laurence's sister has been in Japan since last she saw her.