Driving from Fussen to Salzburg

Half of the route (i.e. Füssen to Rosenheim) is a normal country road, the rest is a constantly crowded highway. I would calculate with at least three hours. If you still have to buy an Austrian vignette, you have to add some time; the same applies if you drive the last kilometers from the border to Salzburg on the country road to save the money for the vignette.

Yes, it is possible at times with little traffic. But if you try it on a Saturday in August, you will need at least 1 hour longer.

As far as Irschenberg you are on a country road, where one or two tractors can slow you down. From Irschenberg to Salzburg you are on the A8 highway. This is very busy and prone to traffic jams.

Before you get onto the highway in Irschenberg, you can make a short detour to this church.
https://maps.app.goo.gl/mDwyCNzHr2ZY3DsR6
On the road there you have a wonderful view.
There are nicer routes to Salzburg without the A8 highway, but they take much longer.

Regarding the highway vignette for Austria:
If you don't have a vignette for Austria, you can drive in Austria as far as the “Salzburg Nord” highway exit.
If you want to buy a vignette, you can do so at petrol stations before or at the border or online at https://atfin.org/en/

I drove B17/B23 from Fussen to Oberammergau. That portion was fantasic. Green rolling hills. Very bucolic. Pleasant where you can drive at a decent pace but not feel rushed. Pretty easy to navigate. Made me regret not spending more time in the area.

Realistically, I would say the whole day. There are so many incredibly beautiful places and landscapes along the way. It's hard to just drive past without stopping.

You could make a day of it (or two) and dip down into Austria and drive the Inn River Valley secondary roads and then towards the Wilder Kaiser mountain. No Toll for secondary roads. I love this in Spring-Fall with the top down. Yes parts can be slow-go and watch for speed cameras.