from my experience it depends on the stream...
I agree with Louis, Shawn.
Mayfly hatches are often, in my experience, pretty dependable, predictable, and easy to identify occurring. I know at certain times of year and at certain times of day what's likely to be going on even before I get to the stream. Can caddis hatches be this way, or are they harder to pin down in terms of times of year and times of day? What factors do they depend on? What do you look for to clue you in as to what's going on?
You are right. They are hard to pin down and not
because of less angler lore being accumulated and shared on the order. In my opinion, the biological factors involved in this difficulty are the reason that there's not as much shared about them to begin with. With some very notable exceptions, most caddis activity is more jumbled and often harder to detect (and hence deal with) than mayfly activity. Here's a typical early Summer day's experience on one of my local rivers I'll pose as an example of what I mean:
First light to 10am - Glossosoma
larvae (Little Short-horned Sedge) engage en masse in behavioral drift. This doesn't happen every day. In fact, it's quite sporadic. I've gone quite a spell before without hitting it. There is no hatch involved so an angler without knowledge of this behavior (or lacking a seine net) is totally unaware. But the fish aren't...
10am till around noon - infrequens
nymphs (Pale Morning Dun) are getting very active. A few duns begin to show alerting the angler to this activity for the use of nymphs.
Noon 'till almost 3pm - infrequens
duns hatch in earnest with ebbs and flows. There are other critters about (a few caddis, baetids, etc.), but not enough to distract Mr. Trout from his focus on the ephemerellids that are everywhere. The fish get very dialed in to this critter and the options can be reduced to a couple of stages and behavior of this single species. Crack the code and it's hammer time.
3pm 'till around 7pm - Activity drops off. There is the odd fish looking for terrestrials or hangover duns and prospecting with flies is still working, but time is perhaps better spent with lunch and a nap in preparation for the evening (which is what I chose to do :)).
7pm 'till dusk - the time for caddis. Herein lies the rub. There are at least 4 species active. A couple of hydropsychiids (Spotted Sedge), at least one rhyacophilid (Green Sedge) and a small dark species of another family thrown in. The day's hatching pupa trickling off are mixing with a weeks worth of females returning to lay eggs; some diving, some dancing on the surface. The air is filled with flying caddis, most of which won't end up anywhere near the water. Which species and stage are the fish focusing on? Are individual fish working on different ones? These are the times when anglers are often reduced to "chuck and chance it", with options hopefully reduced by previous experience, seining, and rise observation. Sometimes you hit the magic formula, sometimes you don't (I had mixed results this night, though two days before under what appeared to be very similar circumstances green soft hackles and pupa really did the trick).
Dusk 'till you can't see anymore - Suddenly the caddis patterns quit working completely and if anything the fish are even more active, though the rise forms have changed. It's too dark to really see what's in the air, so I broke out the seine to discover a spinnerfall of large Siphlonurus (Gray Drake). I was back in business for a few more fish.
Anyway, I hope this illustrated the point I was trying to make.