When do you think is the best time or scenario to start your own business or startup?

I think the question can be rethought with a few tweaks in the parameters used to judge the situation: is this problem exciting enough for me to commit full-time or at least 75% of my time to see how dense and vast this problem is felt globally or locally. And once you're confident the data and market are ripe, the clock has started ticking for you already :)

once you know how to build

In relation to time - You need to start instead of wondering whether its the right time. Even if starting is just a back-of-the-napkin drawing. If you want a good guide to navigate then check out - https://www.100tasks.com/checklist