No one can answer that question for you.
It depends on your prior knowledge, your resourcefulness, your resilience, your motivation and many other things.
Also, it depends on what you mean by ‘finishing’ the challenges.
I have earned the Front End Certificate, but as I learn more and more while working on other projects I realise that I haven’t actually finished! I keep going back to my old projects and tweaking them, and improving them.
To be completely honest, the certificates are not worth much at all. Let’s say you earn the Front End Cert in 2 days. You may have met all of the task requirements, but your apps could be really terrible. If you try and get a job and say ‘Look at my certificate!!!’ but your actual apps are bad, no one will hire you 
On the other hand, there are members of our community that have gotten only part of the way through the first certificate and have managed to land jobs on the strength of their portfolios and networking.
So before thinking ‘What’s the fastest I can do this?’, instead consider why you want to complete the challenges and what true success looks like to you.
That said, it is possible to complete the curriculum much faster than the suggested time frames claim.
You may like to read this, but take it with a pinch of salt, because in all honesty everybody learns in different ways and comparing yourself to anyone else is pretty meaningless unless you know literally everything about them and yourself