I don't think anyone has reached its limit the problem is more likely to get hardware that can support enough RAM... as far as I know most mainstream workstation/server mainboards can only access 12x 16 or 32GB RAM sticks, but some more specialised mainboards can have more ram, above 512GB but you need to have a small country's budget to pay for it.
As Peter said, with 64bit systems you can go into the Terabyte's of ram. 32bit systems are limited to 3.7GB from memory!
All the best and good luck
PS: I use 96GB and that works fine, and I can easily max it out :-)