Hi Sindy,
Estimating course time is a real challenge since, as Richard mentioned, estimates can still be a bit arbitrary. Here are a few techniques I find helpful for getting at least a rough number.
If you already have the content written, you can estimate how long it would take to read it out loud with this website. Many people can read faster in their head than they can read out loud, so you might want to use the fast reading speed if the content will be in text form instead of narration. And if you have any interactions, you'll then need to add time to complete those to your total.
If you don't have the content entirely written, I often list out the main topics/takeaways I need to cover. Then I break down what sub-topics I need to cover and rough out an approximate time for each one. I find these smaller components a lot easier to come up with a rough time estimate for!
If you don't have a time limit, estimate how long it would take to cover each sub-topic well. If there's a time limit (like your project has), you can take your allotted time and decide how to spread it across all your subtopics effectively. When I have a time limitation, I find this helps me figure out fast if I've got more content than I have time for. Plus, you can always use that time breakdown to guide hard conversations with clients/SMEs about what might need to be cut.