you're not gonna like this, but about 3 or 4 years for a commercial quality program.
you don't need to know C OR C++ to write a screensaver, there are several packages available that allow you to make screensavers. It really depends on what you want to do. if you just want a slideshow to show off your photography, that's pretty trivial. if you want to do something really cool like the electric sheep, it's gonna take a lot more work in graphics, math, networking, etc.

In short, if you just want to write a screensaver, stay away from C++.