For me it is very much a relative thing, er, which takes on a double meaning with next comment:
I used to go home to Georgia to visit relatives on a regular basis: every year if it was only a thousand miles or so, every other year from the West Coast (which is more than 2000 miles). But my sister would come out to see me on the West Coast for the years in between. Then I went and took care of her after her mastectomy, sixth months later I was bedridden, and, gee, neither of us has managed to travel cross country in the last (almost) 4 years.
Generally speaking, how long I am going to stay when I get there and how much benefit I expect to get out of it determines how far I am willing to go. Since my commute is from the bedroom to the dining room and I consider leaving the house to get groceries to be a Hassle, it better be Really Worth It to go anywhere that takes a long drive.
So:
Several times a year, I drive 45+ minutes to Sacramento for me and/or my son for special medical services.
I took an 11 or 13 hour bus trip to Southern Cal. (where I got to breathe the smog) for 8 weeks of GIS school (well, technically, for 4 weeks: I came back and picked up my kids and the car at the midpoint break).
We once travelled 5500 miles to move a total of about 1200: when we transferred from Washington State to Southern California via Georgia (where my husband did ANCOC for 3 months and I had the privilege of sleeping on the floor at my mom's house, between 2 kids kicking me all night, and ... anyway, it ended being Worth It, so I won't go into the gruesome details).
And I in May of 2000, did a month-long trip where I flew from Southern Cal. to Atlanta, then borrowed my sister's car and drove to Boston for the Hollingworth Gifted Child Conference, stopped in the D.C. area for a tour of downtown D.C. by my then "boss" when I had a pro bono directorship, back to Atlanta, the next weekend I drove to Columbus, GA, to see my folks and then back to Atlanta where I took care of my sister's daughter while she was in the hospital, then took care of my sister when she came home from the hospital, then flew back to Cali -- all with my 2 sons in tow. That was quite exhausting but also Worth It.
We have also lived in the following places in the following order: met and married in Georgia, went to Texas (about 1000 miles or so), Germany, Kansas, Washington state, then to Southern Cali (via Georgia) then to the San Francisco Bay Area. This last move was a mere 450 miles (approx.) and within the same state. Perhaps God knew I was at Death's Door and was cutting me some slack. :-D
If you slather on layers and layers of benefit so that the cost-benefit ratio makes sense, I am There, wherever it is. :-D