What I'd recommend is setting up a few testing systems with 2-3GB of swap or more, and monitoring what happens over the course of a week or so under varying (memory) load conditions. As long as you haven't encountered severe memory starvation during that week – in which case the test will not have been very useful – you will probably end up with some number of MB of swap occupied.

And

[... On Linux Kernel > 4.0] having a swap size of a few GB keeps your options open on modern kernels.

And finally

For laptop/desktop users who want to hibernate to swap, this also needs to be taken into account – in this case your swap file should be at least your physical RAM size.