Lol, just as a caveat. I'm not actually a microbiologist by profession. Most of my undergrad training was in biochemistry and microbiology, but I ended up getting a job as a materials chemist.
Streptomyces spp. are not nearly as well understood as Bacillus subtilis, but the basic triggers for spore formation are the same - nutrient depletion and environmental stress. Additionally, some aspects of primary metabolism may be triggers as well - e.g., accumulation of glycogen or other energy storage molecules.
You're right that Streptomyces only truly reproduces by spores (called conidiospores in this case). Conidiospores are much easier to kill than endospores, but they're omnipresent in the air and quite successful. Strepomyces is weird because it has some characteristics of a multicellular organism. When it first germinates, it goes through a phase of vegetative growth where it grows a filamentous, branched "primary mycelium" that's really similar to fungal mycelium. Each strand grows only from the tip, and is essentially one cell with many, many copies of the organism's genetic material. Rarely, the filament does actually go through "cell division" by partitioning the strand with a cell wall, but the new cell is not a new organism - it remains integrated with the rest of the mycelium.
When sporulation is triggered, Streptomyces produces an "aerial" mycelium that rises above the substrate and partitions into individual compartments, each with a copy of the genetic material. These compartments eventually become spores, and drift off in the air to find new and exciting adventures.
On a solid substrate, like agar, perlite, wheat bran, etc., sporulation is extremely obvious - a big fuzzy, hairy mass appears. It's usually a darker color than the primary mycelium as well.
It seems that the time to sporulation varies a lot, from 3 days to 14 days. Some of that has to do with the sheer size of the culture involved, but it may take a certain amount of time for the bacteria to develop regardless of other factors.
It's not yet clear to me how you can tell if sporulation has occurred in a liquid (submerged) culture. I'll have to do some more research there.
Truthfully though, it really doesn't matter. Even fragmented pieces of primary mycelium remain viable, and are a perfectly good way of delivering the organism when you're not worried about preserving, packaging, storing, and delivering a standardized product to a customer.
Multiple references do say that producing Streptomyces spores on a sterilized solid substrate is easier and more economical. But it's not clear to me whether or not mechanical aeration is necessary, and how it could be accomplished in a DIY context.
Btw, all of this is a piece of cake for Bacillus subtilis. Just throw it in with some nutrients and air, and let it multiply. Production of endospores, again, really isn't necessary.
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