I realize I was actually just backing up what you said about dosing decreasing duration (I had thought you had meant frequency, sorry for reading too quick).
There are definitely studies that show taking at the onset, as well as consistently dosing, may help decrease duration and symptoms so you are right in the assumption that is probably what you've heard before (with it apparently being conflicting with needing to be taken per-emptively or not).
That said articles (like that NPR one) don't really constitute as evidence though (I guess especially as far as how much stock I'd put into it). Maybe I should write a post on the hierarchy of scientific evidence because I feel like I see this happen quite a bit.
I got bored so I dug into it again for anyone who cares:
Common colds: Does vitamin C keep you healthy?
-This probably is one of the studies that helps highlight the disconnect of maybe what is being said with how people are misinterpreting it. While it seems to not always be replicated and the effect isn't always very large the consensus is that it does seem to decrease symptoms or severity, especially more reliably than decreasing occurrence (I'll link more below).
Where that seems to be getting confused seems to come from two factors (to me):
1) This study shows that that benefit of decreasing the severity and duration comes when taking Vitamin C daily and that taking it once you have symptoms doesn't always help decrease the duration or severity (so it should be taken even before you feel ill, but hold on because there is some information I'll link later that recommends slightly differently).
2) There are actually studies that shows it cutting the likelihood to develop a cold in half, but these are typically seen in very strenuous situations like extreme sport fatiguing scenarios (marathons) or military training (and often in extreme conditions of cold as well). I guess you could argue this means it might be more beneficial for hard training athletes, but I don't think most people have training that qualifies for this (BUT maybe your life stress may make it worth it or during particular training situations).
Vitamin C reduces the severity of common colds: a meta-analysis
-More recent meta-analysis showing it may help with intensity and duration. Still some conflicting information with potential for dose benefit, but just continues to confirm this consensus.
-This one is more helpful for being able to consolidate so many studies and give a place to pull more from when needed.
Vitamin C in the Prevention and Treatment of the Common Cold
-I am starting to feel lazy so here is just a direct quote:
"While regularly administered vitamin C did not seem to affect the incidence of the common cold in the general population, the review also analyzed 31 comparisons on the effect of regular vitamin C supplementation on symptom duration and severity of colds. The results indicated that regular vitamin C supplementation (at an average dose of 1-2 g/day) resulted in a significant reduction in the duration of common colds, an 8% reduction for adults and 14% reduction for children. The severity of cold symptoms was also reduced. "
-This one also mentions some dosing, but as with many of the other studies one of the difficulties falls to the fact that studies use different measures of what they consider symptoms and severity so it may be difficult to precisely assess exactly what can be recommended and assumed for how effective it may be.
-Highlighting this one due to the fact it did show benefit to dosing after noticing symptoms, but must be done within 24 hours and for 5 days duration for most likely benefit.
Vitamin C for preventing and treating the common cold
-I'll probably stop here because many of these will just be highlighted already in the meta above if anyone really wants to dig, but this highlights a few points I think are worthwhile.
-Again going to just copy the conclusion:
"The failure of vitamin C supplementation to reduce the incidence of colds in the general population indicates that routine vitamin C supplementation is not justified, yet vitamin C may be useful for people exposed to brief periods of severe physical exercise. Regular supplementation trials have shown that vitamin C reduces the duration of colds, but this was not replicated in the few therapeutic trials that have been carried out. Nevertheless, given the consistent effect of vitamin C on the duration and severity of colds in the regular supplementation studies, and the low cost and safety, it may be worthwhile for common cold patients to test on an individual basis whether therapeutic vitamin C is beneficial for them. Further therapeutic RCTs are warranted. "
Overall I think that helps highlight many of the current state of the research. Some issues just tend to be trying to turn how scientific studies use words to say something with how you can interpret that in the real world. I'd say based on all this:
1) Take it daily since it is cheap and may possibly help decrease symptoms (with some saying that you need this daily dosing to get the benefits, as in not just once you feel sick).
2) Maybe take more during particularly stressful times and/or bump that dosage up (even to like 8g+) when you feel sick (almost like hedging your bets, take some daily then might as well bump up when feeling sick or stressed to see if it personally provides help). Keep in mind to spread the dosage out as too much all at once can cause GI distress.
Sorry for slight derailment, but that might be helpful for anyone as I know this topic does come up from time to time.
