It can vary on the current settings inside the plugin, so I would expect it can only be calculated when Sonar begins playback. But I agree, this would be a very useful thing for Sonar to report. I'd vote for it.
In theory I would expect modulation, delay, gain, and saturation effects to have zero or near-zero latency, EQ to have moderate latency, and convolution effects (cab sims and reverbs) to typically have large latency - purely based on the sort of operations they need to perform. But so many effects these days are hybrids of the above that it's hard to be sure.