Yes, MagicSmoker's general assumption of heat transferring sideways approx. 15mm is a good one. Let's round it down to 10mm for now, for a bit of extra margin: assuming imaginary component of 10x10mm, this means you have dissipation area of approx. 30x20mm, assuming one side of the component is needed for tracks.
Now, adding a large bunch of thermal vias - certainly within that 10mm, but the closer the component, the better - allows you to continue heatsinking on the other side. Then, you can go 10mm both ways from the vias. If your vias are right say 1mm from the component, now you have 30x30mm fill on the bottom side, as well.
There are web calculators that calculate the thermal resistance of the via for you! So you can just divide by number of vias, to get an estimate how much cooler (and hence, less effective) the bottom side fill runs.
If you need more heatsinking, this bottom side 30x30mm fill is the place to couple the actual heatsink to. You have a surface area approximated (say 30x30mm), you have the thermal resistance value and thickness of whatever thermal pad material you are going to use, so you can calculate the total dT.