The sky is quite literally the limit with Gabapentinoids, or at least in my experience Gabapentin (Neurontin) and Pregabalin (Lyrica) specifically. First off, for most people Gabapentin has a very wide therapeutic index compared to other drugs. Overdose is not an imminent threat unless extreme doses are taken, although some people do experience convulsions when excessive doses are taken (myself included). Tolerance builds so quickly with this drug that it's practically exponential.
Here's an anecdote concerning my own use of Gabapentin, just for reference. I pick up a prescription of Gabapentin (800mg x3 daily, 90ct) and ingest my normal, recreational yet functional dose of ~3,500mg. I get the desired effects, everything is groovy, now, I wake up the following day. I know from experience that I will need to escalate my dose by 50% to achieve the desired effects, so I do, I take approximately 6,000mg and I still am not where I want to be. I was dope sick so I was really relying on the Gabapentin to help me through. I take 2,000mg more. On the third day, I find I need about 10,000mg - 12,000mg to alleviate my withdrawal symptoms in a 24 hour period.
Just continue going from there and I think you get my point. Tolerance is the calling card of Gabapentinoids and is actually, in my opinion, a real barrier to their continued abuse, because I for one really, really enjoy their effects, yet really can't use them regularly for these reasons.
So really, like I said, the sky is the limit. If you let things get out of hand, you can easily devour your 30 day prescription in just a few days. I've done it and I know plenty of other people who have. There's also a girl at my Methadone clinic who is essentially addicted and dependent upon Gabapentin/Pregabalin. She spends up to $50 per day on the stuff, which considering the realtively low prices, is a large quantity
30g - 40g of Gabapentin or equivalent in a severely addicted and out of control person wouldn't surprise me one bit. I'm sure this sounds outrageous to some, but I'd be willing to bet there are more than a few people in this range. Not to mention the stories you hear about Phenibut, which in my experience is at least somewhat cross-tolerant with the former two drugs, in which people who have a near unlimited supply of the stuff end up in uncharted terriory with how excessive their doses are.