I would say "nearly all of it", as in nearly all of the useful land (as defined in the poll). That will probably leave about 10-20% of the map unsettled, depending on how much snow is on the map and how many one tile islands spawn.
For me, it's more of a pragmatic concern. Land grabbing means the land is worth grabbing, which could be good for balance. The exception to this is if it's not land that is important, but if cities themselves are somehow inherently valuable, regardless of where they are settled, then that's a problem that leads to an ICS-like mentality, which would most likely result in the entire map being claimed. I prefer an approach where it's the land that is valuable, and it's valuable enough to allow you to offset whatever restrictions are designed to pace expansion.
If the land just isn't settled, though, that implies it's not worth settling, which is imo bad for the game and actually more of a concern than some may realize.