as a project manager or CTO how would I calculate the "time" it takes to develop a software system? For Example I would be responsible for developing a large web application in Java, MySQL, HTML, JSF. How do I correctly calculate how much time this takes?

Prerequisites: I do not know the team that will do this job (so I can not ask them how long they might need). I did not do any calculations like this before. I am completely new in this role. But I have some very basic/slight programming background...

Also any good resources (=links) or answers are very much appreciated.

Thanks very much! Jens