Pleased to meet you Hien, my name is Carla and I’m from South Africa 
I think 5 months is a fantastic age to start your little one’s EL journey and I’m sure you are very excited. The questions you’re asking are very tricky, they depend on so many factors. Your child’s personality and how much he enjoys his lessons will greatly influence his learning. I chose to show the TW lessons a total of 8 times each, that is roughly 2 times the first week the lesson is introduced, then about 2 weeks later, then a month later, then 2 months, 4 months, 8 months, etc. We’re only watching 1 episode a day. This choice was based on my son’s age (he was about 2.5 years when we started) and the fact that a lot of the material covered in the lessons was old for him. I started including TW in our days because I was after the right brain stimulation it gives.
The Shichida method encourages us to only show lessons (particularly image flashcard and EK) 2 or 3 times in total. The fast flash and limited repeats encourages photographic memory. But when it comes to material that you would like your little one to memorize and become very familiar with (like phonics, sight words, quantities or dots, etc.) you should probably take Glen Doman’s advice and repeat many many times (3 times a day, 10 to 30 days is what is suggested when you start out with Doman’s bit cards.) You can use this knowledge to start off with, but from here you’ll have to pay close attention to your little one. When he’s older he might display his knowledge (take note that children usually dislike being tested and many seem to hide their knowledge for months or years before you see anything,) but while he’s this young, you can try and tell by how he responds, if he use to seem interested in the program, but he starts to loose interest, that might mean that he’s learned all he wanted from that DVD and he’s ready to move on.
I think many of us stop caring about if a product is working or not, because we’ve got many backups. Most of us use multiple things, from apps to books to videos, I don’t mean to overwhelm you when you’re just starting out, but over the course of the following months and years you’ll probably accumulate a lot of EL stuff too. I’m not saying get them all, TW is already a very complete package, but it’s nice to have some variety and you’ll find many programs compliment each other greatly.
Something else that you should keep in mind is that, you’ll make it easier for your little one to learn from the lessons if you incorporate some of it in your daily lives. At 5 months it’s harder, but if you have alphabet letters of some form (fridge magnets, foam letters, soft blocks etc) you could play specifically with 2 or 3 of the letters that were in today’s lesson. If they were counting by 2’s, then count some toys by 2’s. This is not as important when teaching a baby as it is when he reaches 2 or 3, but it’s good to start getting into the practice of doing so. Again, there’s no need to go overboard and replicate the entire lesson in real life, just a few things to show the baby that the things in the lesson matters to you and it exists in his every day life. An extension of this idea is to put posters, pictures or word labels up on the wall in his room or anywhere where he could see it daily. You could point to them, but even if you don’t he’ll be seeing it often and his curiosity will grow, therefor his enjoyment in the lessons will grow too.
I was always one to complain on the forum about my boy not learning from our activities, now finally after 3 years, more and more knowledge just seems to come to the foreground! It really isn’t easy to wait for your little one to start reading and doing math and all the wonderful things we see on the videos, and of course it is a possibility that your own baby might be one of the many wondrous little ones who read or do math and music early, but this is by far not the norm. EL has huge benefits in the long run, that should be your motivator, but placing too much hope on an 18 month old reader is setting yourself up for disappointment.
Please try not to think about if he’s learning or not, it’s unnecessary stress. Small children and especially babies don’t need to look like they are concentrating for them to learn. Many here on the forum have reported that they taught an older child something, then when it’s time for their younger sibling to learn that skill, he basically already knew how to do it, and then they go on to say that he never even looked like he was paying any attention to it.
Sorry for not giving you a straight answer, but I really don’t think there is one.
Good luck and enjoy it! Teaching my boy is the most rewarding experience of my life!