In our experience, most language teachers go into a classroom once a week for 45-60 mins. When you consider all of the other things that go in during a school week, it can be challenging for students to retain the information that they have learned from week to week. If there is an interruption, this can mean that you might not see your students for two weeks. This is one of the reasons we recommend changing topics regularly. I remember planning a 6 week unit of work on Animals for my Indonesian students. By week 3 they were over it! When I began teaching languages, I was using way too many worksheets and there was a lesson once where I did not double check my resources. I think it might have been on animals as well, and I had the vocabulary written down for some of my tasks, but one of my worksheets had a completely different set of vocab which I hadn't realised. Due to the fact that I was thrown in the deep end a bit, and I wasn't fluent in Indonesian, I didn't know some of the words that I was expecting my students to use to complete the task... Massive lesson learned for me! Another tip - carry a dictionary at all times!!! It is ok to tell your students if you are not sure what a word means, or tell them that you will double check in the dictionary. Checking the resources you are using and writing down the vocab will ensure you do not run into this problem. Mini student dictionaries are great if your budget allows for this.
I also can recall a time in my first year of teaching when I spent 7 hours preparing a lesson that they students completed in 40 mins. I remember feeling so frustrated that I had spent an entire Sunday creating resources, doing research and putting together some tasks for my students, all for them to get through really quickly. I also didn't laminate the game cards that I had made - so they were ruined!!
When we prepare our lessons now we follow a few key steps. 1 - select the topic. 2 - select the vocab around that topic. 3 - choose games and activities where students will be using the vocab 4 - try to incorporate the following for your students - speaking, reading, writing and listening skills.
This is a very simplified way of looking at things but it is a good place to start!