Not very much - you want to have atleast 1Gbyte ram for every 100gig of cache Otherwise you will have poor performance, and even crashes (atleast this is the case with squid)
The caching proxy feature of RouterOS is meant for small networks and a couple of users. I you need a large caching proxy, get a dedicated device that runs software explicitly designed to serve as such.
There are power routers by linktechs(Mikrotik OS) out there that do a great deal of cache than what you have stated and handle quite big networks we would say 300-500 concurrent users and more bandwidth