In the larger anime cons like AX, it's almost
impossible to get a sketch from any of the guests.
Anime Expo uses the lottery system which only
like 10 out of several hundred people will get
the sketches, then 50-100 will get signatures.

In the smaller cons, sometimes, the artists will
draw most people sketches if the lines are short
and not using the lottery system like Anime Expo.

And in certain cons, some artists are invited by
certain companies instead of the conventions, so
they sometimes have their own signing sessions in
certain companies' booths.  

In Japan, the situation is different.  The 
established anime or manga artists rarely draw 
sketches for their own Japanese fans.  And they
don't even go to any convention in Japan.
If they do go, they only do signatures, so the 
sketches have high values and demands in the 
Japanese market.