Tom, I've no idea about S Dakota, but have been watching ruby throats here in Missouri for several years. Here's extrapolating that, along with a few bits from the books:

Hummers arrive here at the end of April. You first see them here around the time the dogwoods/ wild honeysuckle bloom early in May. They are definitely out there in May, but females are nesting and very elusive because they are trying to keep their eggs/babies warm in the cooler temperatures. Around the beginning of July, when the agastache bloom, there's more activity. I think that may be when the babies finally fledge.

As you say, they are scarce early in the season. I asked myself the "is it worth it" question every May for several years, but then I would happen to see one or two. So I decided it is worth keeping at least one feeder going near the house where it is easy to keep it up if for nothing more than to help the mother keep her young alive. May is a hard time for the nesting mothers because it's often too cold for them to leave the nest for food, and some have reportedly starved to death trying to keep their babies warm. So I use a stronger solution and keep the feeders up in case she needs a little help.

I usually see one several times a day from early May on. With the cooler days you don't have to change the water as often. You are more likely to see them right at dawn and at dusk when they stoke up for the night. They do favor feeders that are near (may 8-10 feet) away from a tree or large bush.

If I were frantically busy with work or with children I might not bother. It's your call, but they really are out there even when you happen to not be there when they come. Fussing with the feeders every three or four days in the cooler weather is even kind of pleasant, takes only a few minutes. Try to notice what's blooming when you see them, and that will give you clues about when feeders are needed.

I don't know if it's the *same* birds all summer, because they look so much alike and you only see them still for a few seconds at a time, but there are some, at least a few, out there from the beginning of May on. Also, the books say that for every one you see at one time, there are 5 others out there.

For sure you want a feeder up by August. You should see activity right away then.