How much effort does it take to create your own custom Linux OS / launcher?

How much effort does it take to create your own custom Linux OS / launcher?

For example: if I wanted my own gaming platform (basically: a glorified PC) that ran on a custom Linux OS / launcher, how much effort would something like that be?

I understand that one would most likely be looking into hiring coders, but it would still be nice to have some sort of idea (in terms of hours) of how much effort would be involved in creating a Linux OS / launcher for such a platform.