Well if you had supplied a log (as explicitly stated when you join the forums), we might not have had to ask so many questions. In fact we still don't know enough to give you a definitive answer, but I'll bridge the gap with a long write up:
Okay, so you're recording at 50mbits/sec and have 16 Gigs of RAM, but can upgrade to 32?
For the sake of these calculations I'll assume that you have 10 Gigs of memory free:
10GiB*1024=10,240MiB*8=81,920Mbits/50Mbits a sec of recording=1638.4 seconds/60=27 minutes of replay buffer.
If you had 32 Gigs, with 26 Gigs of free memory:
26GiB*1024=26,624MiB*8=212,992Mbits//50Mbits a sec of recording=4,259.84 seconds/60=70.997 minutes of replay buffer.
Now you don't say if you're using Studio or Classic, but I don't think the replay buffer is available in Studio at this time, so that means you're using Classic (which would have been good for you to state upfront, or post a log so we could know).
For me the buffer stops at 1800 seconds regardless of running 32-bit or 64-bit OBS, but as noted above that would be over 27 minutes of replay buffer (actually 30 minutes on the nose).
So what's the issue? Are you using the 32-bit OBS instead of the 64-bit OBS?
Again, we would know if you posted a log as instructed by the READ HERE FIRST post at the top of the forum.