surely there must be a distance at which it's not worth going up to 35,000 ft because of what it cost to get up there both in fuel and time. I just wondered what that distance was, approximately?
Same answer as before -- it depends on the airplane. Some won't get to 35,000' at max load; some will have a higher cruise altitude.
The minimum practical distance to climb to optimum cruise altitude for a given configuration is the distance it takes to climb to that altitude on a normal climb profile (again, varies with airplane) plus the distance it takes to descend on a normal profile (commonly, distance in NM = altitude in 1,000' plus 10 NM). Again, because of engine differences, some airplanes may benefit in total burn with the climb followed immediately by the descent, while others may benefit by 10 or 20 minutes at [a slightly lower] cruise altitude.