I think for DOS/Win3.x it's 256mb.

For Windows 95 I'm fairly sure it's 480mb.

For 98 SE it's 512mb, up to 2gb if you're hackerman.

On 95, 98 and 98SE there is a bug in VCACHE.VXD that causes it to use up too much RAM for system cache when there's more than 512MB of RAM in the system, to a point where there's not enough room for the OS itself in System Arena, causing a cascade failure. Once the file is patched you can go a bit over 1GB before stuff breaks down again (I no longer remember the exact reasons). MaxFileCache needs to be set to something like 128MB or less or DOS boxes stop working with "not enough memory" errors and other oddities. There's also a commercial patch that allows 2GB without any extra headache if you got 20ish usd to spare.
WinME goes to almost 2GB without any tweaks.