Q:

What is the speed of the Force?

For instance, if a Jedi Force-pushes something 1000 meters away, is the effect instantaneous or is there a propagation delay, something like the speed of light?

A:

The same as the propagation delay for hologram transmissions; presumably instantaneous
As mentioned in a related question, Darth Sidious once Force-choked Dooku from "light-years" away; the hologram of Sidious and Dooku's asphyxiation were perfectly in-sync, which means Sidious' Force use propagated no slower than his hologrammatic self did.

Since holo-conversations (such as many conversations assorted people have with Sidious, or with Leader Snoke in The Force Awakens) don't generally appear to suffer from any kind of lag, both are presumably instantaneous.
It's not immediately clear whether this would apply to Force abilities other than choking, but there's no good reason to suspect they wouldn't.

A:

It depends on the power
In the films, we see Yoda, Obi-Wan and Leia reacting to far-away deaths instantaneously. There is also the transgalactic Force choking mentioned in Jason Baker's answer. These seem to indicate that there's no speed limit on the Force itself.
However, other powers (such as Force push, lightning, etc.) seem to happen as a "wave" (see 1:43):

So for some powers at least, it seems like there is a delay.

