I just did a little experiment in gta 4, starting out in Alderney city on an NRG - 900 I rode up onto the freeway and around the entire outskirts of liberty city all the way to my original starting place, it took 15 mins more or less, minus a minute say for the ocassional times I fell off and had to pause to set a new waypoint. So if Los Santos alone is bigger than liberty city plus the rest of the map then it's pretty clear to me how big it is going to be. It's gunna be huge
The other day I circled the whole LC map in a speedboat, non-stop. Took a full 12 minutes.
Again, if LS is the size of LC and circling it can take between 10-15 minutes, circling the whole map (which is about 3X the size of the city by the looks of it), should take somewhere around 30-45 minutes.
If the map was entirely flat it would take 15 minutes just to cross it one side to the other. And this using, in your case, a motorbike and in my case a speedboat. Using a taxicab, over the real terrain (which is hilly, even mountainous) using the actual road network (which won't be straight), would probably take twice as long.
Basic maths tells you it's a good twenty or thirty minutes to get from LS up to Chilliad even in a moderately fast car. People forget that the terrain is hilly and mountainous and that DOES add to the terrain and the breadth of the map. It's not empty terrain, it's going to be full of interest!