Illegal workers are not just Mexicans in America. It is also Americans, Canadians, and others working in Mexico illegally to support their “indefinite stay”.

Before I had residency status my wife and I came to Mexico for eight weeks one time. (I had a lot of vacation to burn off.) When we arrived we had a typed itinerary we had drafted and I brought several of my recent pay stubs with me to alleviate any concerns migración might have had. We planned to be in Mexico for eight weeks. We asked for 10 weeks; we got 180 days.

After arrival our plans changed and we travelled to other places, but I think that showing our financial independence and having a plan helped.

Upon arrival to Zihuatanejo they ask you at customs how long are you staying.
What if you don’t know that and your travel is open ended.
Is there a problem when you depart or how do you answer?

You are there with a tourist visa, yes? My layman understanding of the law is that you can stay up to six months. Tell 'em the truth that you don't know but you will be sure to obey the law and leave Mexico within the six-month time limit.