I don't know where you can check it on a specific Linux system. I just know that 3GB is typical and is the value on every 32 bit Linux I use. I'm pretty sure it can and sometimes is adjusted as a build time option.
32 bit mode has a 4GB limit per process (including kernel) in the hardware architecture. An OS design must divide that between kernel and process. Windows by default divides it 2GB/2GB but can be set to divide it 1GB/3GB. Linux typically divides it 1GB/3GB, but can be set to 2GB/2GB or to (inefficiently elsewhere)/4GB.
The current hardware architecture for 64 bit has a limit (per process including kernel virtual) of 262144GB, and is designed to make an even division between Kernel and process most practical. So an OS could easily have a limit of 131072GB virtual for each process.
However, there are some performance advantages to designing the OS with a lower limit on per process virtual memory. I think I read somewhere that XP64 set that to 320768GB per process virtual. I think there would be a significant simplification and performance advantage at a limit of 1024GB process virtual (but that would actually constrain some plausible uses, so it wouldn't be a good idea). I don't recall what if anything I've ever read about what Linux has done for that.
People asking these impractical questions are usually referred to
as an introduction to where you might look to find it yourself. Just now, I followed a few links there to see if your answer was given. I didn't find it.
BTW, the x86_64 architecture (unlike the 32bit architecture) is designed to allow a future expansion far beyond 262144GB involving a change to both CPU chip design and OS kernel source code, but NOT requiring any user code to be changed nor even recompiled.