Guidelines for allocating system resources for VMs?

[Albin Otterhäll <gmane@otterhall.com>]

I want to use virtual machines for my everyday work on my laptop (with a
Intel Core i5-3320M @ 2.60 GHz * 4 and 16GB RAM), using KVM on Debian as
my hypervisor.

But I can't find any general guidelines for how much system resources to
"give" to a VM. How many logical cores? How much RAM? Note that the
primary OS only should act as a hypervisor.

I will primarily use a Debian VM (i.e. Debian VM on Debian), but I also
want to be able to run a Windows 10 VM (Windows VM on Debian). It's not
necessary for both VMs to run simultaneously.
-- 
Regards,
Albin