Yes! For those who are sensitive enough, you can feel ki on day one.
It depends very much on whether the teacher understands ki or not.
There is nothing mystical about it, ki has been used as a concept for thousands of years, in virtually all cultures.
The teaching of ki changes students' aikido from merely mechanical movements to graceful flow. Obviously this takes time. I have found that, while a beginning student can feel ki on day one, it takes several months or years to understand it and to practise it. It is a felt understanding, not one that can be learned intellectually over a weekend.
Thank you for sharing, I can see it has triggered a variety a responses!