I don't think thicker than 1/8", or even that thick would be necessary. You just need it adhered well to the surrounding area, and if you vee out the crack, that will help with adhesion and sealing. Like has been mentioned above, prep and cleaning is very important.
Thicker applications do cure faster, but that isn't really an issue or an advantage, as it's a chemical cure. You do need to keep the block warm, may be put a radiant heater on it before and during curing. If it's too cold it won't cure well.
