Don't know about timing, but I had one in June. Then Saturday, there were three. Fortunately, parasitic wasps had done their thing on two of them.

I am killing about 2-5 a day on just 3 plants for the past 2 weeks. But I live in the big riddled humid hell of NC

I haven't had a single one all year! It may be because I was very diligent last year. I had a bunch of volunteer toms that came up far away from my garden, so I thought that I would experiment.

When I found one with parasitic wasps on it I moved it to the volunteers. Then each one I found afterwards I moved it there also. Within days they would have parasitic wasps on them.

I'm not sure if that helped or not, but it made me feel good!

BT is the best. I don't think I would have had one plant after the Spring because of those little brats. Spray it on the veggies only, and before you see a mad breakout, and you should be fine I think!

You can look for waste to help find the worm & pick it off.
The moth comes out in Spring in S.C., then lays eggs on tomato plants. The worm eats his fill in a few days & falls to the soil below , then digs in. We are blessed with a long growing here & the worm become a moth & lays eggs again in July. So I get two crops of THW, but I only got 6 worms this year. Mostly because I hand picked them, tilled in the fall to kill most of the worms in Pupae form & rotate my crops.
The wasps (2 kinds) help a lot.
Trap plants help also.

Here is a link that might be useful: Kill Tomato Horn Worm