I've seen them as early as July 4th. Standing in the river covered in white flies watching the fireworks over the river...
It also depends on location. In some spots the hatch can last over 3 weeks and in others it's concentrated and blows up in less than a week, even over a short relatively short section of river (from Duncannon to Wrightsville).
I was at the river on the 13th and 17th and both days there was a moderate hatch. Almost no fish working, but the water was a little higher than ideal for fishing the hatch. Based on what I observed, I wouldn't make a special trip to fish the hatch if it's long drive for you. I'm only 15 minutes from the river so I might give it one more try. I saw one carp, a couple rock bass and I think one smallie. Missed one rising fish; caught nothing. Not surpised.
Now that it's dropped back down to a more seasonal level the fishing might be better. I've found the fishing is always better if the flow is below 10,000 cfs at Marietta or below 8000 at Harrisburg. Lower is even better.