That's a tricky question. There are some factors that change the answer. Like which render engine? What are your render settings? Like samples, and bounces. Is there volumetric? Hardware specs? Will it be 1 light per pass or a few lights?
Really the only true way to know is by testing it with your specific setup. In general it will take longer to sperate the lights into separate passes, because of how Ray tracing and path tracing work. Most of the render time is on the calculation of rendered objects and their materials. Of course emission materials in cycles counts as lights. If you have a large(relative to your hardware) number of light like 50 or more and low end hardware then it might help to break up groups of lights into passes.
