Usually, I wait until the fruit has dried or mostly dried, but this year's weather leaves me with plump, firm, red, peppers on the plants. Temperatures remain in the 40s for highs, around 32 for lows, and we had snow a couple of days back.
Can I harvest and air-dry them and still expect viable seeds?
Leave them for while. We have lots of hot peppers, although we buy our seed. We plant in a different area every year. Every year pepper plants pop up where we had them the year before.
A few years ago we had some Thai Hot Dragon plants. We pull them up & sell the dried plants. The next spring we could not find seed of them, so we took one pepper off one of those plants. We got about 40 plants. Not one of them looked like Thai Hot. But they were some of the best peppers we ever raised. Each plant was different, so we sold a mix of them we called Super Fire Mix. Of course the next summer people came looking for them again! We can't duplicate it.
Just gut the peppers onto a paper plate or coffee filter. The paper breathes and helps draw out the moisture. As they dry, I help them come off the core because it takes forever for that to dry. After a few days they'll be ready to bag up. If you want to save seeds from something very small like Tepin, just dry the whole ones and plant them. I keep one over winter and just push a few pods into the soil with their mama and they come up when they're ready.
As long as a pepper is ripe, the seeds will be viable. They do not have to be over-ripe. In fact, some pepper experts suggest harvesting seed peppers at peak ripeness instead of past. Either way will work though. As long as the pepper has changed to its final ripe color, you'll be fine and your seeds will be mature.
The only problem I have ever had with saving pepper seeds is that they never come back true. I only grow O/P heirlooms but they X pollinate readily. Right now I have purple Jal planted next to some N african varity that is red. Some of the purple Jal. are red and some of the African reds have purple on them. I'm planning on saving some seeds from both to see what each one grows back as, because genetically they should be the same.