Hi, Heather,
It always brings back a lot of memories to read posts from people planning their first Caminos. Lucky you. All the unknowns are such an important part of the experience, and those of us from the US or other far away places just have to deal with some details that others don't, as far as travel goes at least.
Here's what I usually do, it's a slight variation on what others have said. The first thing is to decide where you want to spend extra days if you in fact have them. If you had decided you would spend them in Paris, your point of departure for the US, then you're going to have to play it by ear and wait to see what unfolds. But if you know you want to spend them in Santiago, or walking to Fisterre or Muxia if time allows, then you can book your return trip to Paris now because you know you'll be hanging out in Spain if you're done early. And since the date of your return trip to the US is, I'm assuming, fixed, you can just figure out when you need to be there and book accordingly.
I usually come in and out of Madrid and usually want to spend one day in Madrid before leaving, so I always book a flight or train from Santiago for the day before my departure to the US. And then I just wait to see how much extra time I have in Santiago. Last year I was able to walk to Fisterre and Muxia and it was wonderful, but sometimes I arrive with a few hours to spare!
And I agree with the others who say that six weeks walking time is generous. Though there are huge ranges, 33-35 days is a pretty typical time frame. Especially for someone like you, whose picture suggests that you must have spent the last few years running after active small children! Buen camino to you, Laurie