Actually no need to boil it at all. Shop-bought milk will already be pasteurised, so no need to heat it again.
I just take my fresh-from-the-goat milk (no pasteurisation or any other faff) warm it till it feel warm to my finger, add a blob of commercial yoghurt (usually Yeo), and off it goes into my yoghurt maker, left for 10hours. Quite runny but delicious...
At the moment we are just making kefir all the time, even less hassle...
Pasteurisation just kills all the good bugs in the milk, what's the point (for home consumption) of it exactly?