Imagine an "average" Third Party Publisher in terms of size and, perhaps more importantly, number of associates of that TPP who are willing to playtest for it.
I can't imagine it because I don't know what you mean by "average". Are there even enough TPP out there to enable such an "average" to be defined? I mean, most TPP don't produce more than one product, period. So who does that leave? Critical Hit, Schwerpunkt, Bounding Fire, Le Franc Tireur, Friendly Fire, Lone Canuck ... am I missing anyone significant?
Critical Hit are irrelevant to the discussion because irrespective of their production rate their (new) products are badly-designed and not playtested by anyone.
Schwerpunkt rarely ever offered more than 2 scenario packs in a single year and these days are unlikely to even do that.
Friendly Fire never do more than one scenario pack a year (and not always even that).
Which pretty much just leaves Lone Canuck, BFP and LFT. If those are the guys you want to talk about, why aren't we talking about those guys specifically?
What rate of publication for that publisher would make you start to wonder if its goals were beginning to outstrip its realistic ability to fully playtest and develop its products?
I've never thought of a TPP in terms of their "rate of publication". What I think mainly about is: do I want to see publications like the ones they are doing; if I decide to purchase, did I feel that I was getting a quality product; and how much did I have to pay to get it. And that's all about individual products, not a question of how many products there are in total.
Would more than one product per year start having you a bit concerned? (that is when I personally tend to start having doubts) More than two products a year?
As I indicate above, I don't think in those terms, so the questions are irrelevant.
Do you think a TPP can actually release three fully developed and playtested products per year?
Can they? Absolutely! As indicated in previous replies, there might be very good reasons why a single TPP might deliver multiple products in a year, and if they were all worked on independently and separately they might all be excellent.
However, I think what you actually want to ask is, can three products
all worked on by the same people be developed and published to a competent-or-better standard in a single 12-month period? I would suspect that to be a much more difficult trick to pull off. I certainly wouldn't declare it to be "impossible" though.
Do you trust that a TPP that released any number of products per year was fully playtesting and developing them beforehand?
The proof, as they say, is in the pudding. Was I happy with the previous products that were released? I'll keep "trusting" a TPP to maintain (or improve) their standards until I see direct evidence that they clearly aren't.
Have you ever thought of any past TPP, "Man, they are releasing too many products too fast for them to really have fully playtested them"?
No, I don't think I've ever wondered that.
Since we've already established that we're only talking about three TPP in particular, I can't give you specific answers for those three because I no longer purchase any products from those TPP (in fact there are not too many TPP products that I purchase at all these days). The question of "quality" is really only relevant to one of the three (and even for that one, is only a part of my non-purchasing decision); I have alternate reasons for not purchasing from the other two.