As I mentioned briefly in my app, Shinji kind of has a drug problem. The extent of said problem ... gets into pretty spoilery territory, so I'm just gonna cut this whole thing and spare people's eyes. This post also involves a lot of speculation which may or may not actually get into play; it depends on who I happen to be threading with, and what gets played up more, and what I end up choosing to play with as I settle in with him more.
Shinjiro is addicted to some bad, baaaad stuff. According to Takaya's comments throughout the game, these are Persona-suppressing drugs that really and truly fuck you up real good -- to the point where all the members of Strega are dying from the side effects, and it's heavily implied that Shinji is in the same boat as them. Said suppressants are probably addictive, with nasty withdrawal symptoms. Chidori goes without them for about five days before her Persona starts attacking her; she is then kept in the hospital until Strega decides to come and break her out of there. Shinji's probably better off re: the suppressants due to the fact that a) he was not manufactured the way Strega was, b) he hasn't been taking them as long as Strega has (though this is probably a double-edged sword -- I'll come back to that later), and c) his initial loss of control is probably due to circumstances that are not likely to repeat themselves (although the fact that he knows exactly what to do when Chidori has her fit probably means that he's lost it a few other times as well, mostly due to the drug use). Still, suddenly getting off any drug that you've been taking for two years, especially if said drug is already known to have lethal side effects... that ain't gonna be pretty.
And it's gonna have to happen eventually. Considering that the drugs are experimental, they're probably not going to be replicable in camp and/or they'll need a crack substitute. (I'd prefer to detox him at some point, so option A is going to be my choice for the moment.) There's also the fact that Akihiko knows he was taking the drugs before he died, so when Akihiko finds out, I sincerely doubt Aki's gonna let Shinjiro keep taking them. (For good reason.) I have no idea how often Shinji gets his supply of drugs from Jin, but it's probably relatively often, considering how casual the whole transaction is. Even so, Shinjiro has to get more than four months' worth of pills at a time. He gets a refill on 7/25, starts sharing drugs with Chidori on 9/10, and presumably shares them with her up until his death on 10/4, with absolutely no on-screen fear about when he's gonna get his next fix. Month and a half single doses + one month shared (i.e. twice the amount getting used up) + at least one month buffer period so that he's not concerned about drugs just yet = five months worth of supplies. I'm gonna round up and assume that he got six months' worth of drugs from Jin that last time, meaning that he has about two months camp-time to still be on the suppressants regularly.
Lifespan. Thaaat's a good question. Takaya implies that he wanted to "see the end" before he dies. Considering that 1/31 is ~4 months after 10/4, his lifespan is probably somewhere around 4/5 months. Even though Shinji doesn't have the same origins as Strega, the fact that Takaya mentions that he doesn't have long to live (before killing him) and earlier comments about how Shinji's in the same situation, I'm gonna assume that Shinjiro has about 4-5 months left as well. On one hand, he wasn't manufactured the way Strega was, and he hasn't been taking the drugs as long as they have. On the other hand, he has been taking them for a while, and it's possible that not taking them from the very beginning would be harder on his system. Assuming that it all balances out to 4-5 months is what I'm gonna do.
As for how he heard about the drugs / where STREGA's been getting them ... heck I dunno. Akihiko seems to know about them when he confronts Shinji on 9/10, but his knowledge is vague ("I've heard about those. They're taken to suppress a Persona when the user can't control it. But, the side-effects..."). Shinji says he learned about them on the streets, which is possible, but I think he probably heard about them before leaving SEES. He doesn't strike me as the type to randomly run off unless he has some kind of plan in mind. He probably heard rumors while in SEES and ran off to confirm them after the first October 4th incident. Supply ... uh. I dunno? Strega says they've been taking the medicine since forever (and that it caused the deaths of 97/100 people involved in Ikutsuki's experiment). Ikutsuki couldn't have been giving them drugs directly, but I imagine Jin was smart enough to replicate the suppressants according to Ikutsuki's original design. They then pass the drugs onto Shinji.
... la la rambling. ANYWAY. This is more of a personal reference for me, to keep these things running in the background. Normally it wouldn't matter, but. Uh. CFUD is all about the longevity, nowadays. It's something for me to think about later down the road.