It happens that misterroy formulated :
I think the traffic lights here are just set on a timer and the sensors not
used, and the timer is always way way longer than is needed to clear the
lights.
So if I am approaching a set of these lights at red, and I am the only car on
the road, how long should it take for them to show me a green?

Long enough for the lights at the far end to change to red, plus what
ever time it might take a slow vehicle to make it through and clear to
your end. It might be much longer, where the lights include a junction
and an extra road.