Beating eggs & sugar when making gelato I have attempted to make coffee flavoured gelato.  I have run into a problems specifically when the beaten egg yolk/sugar mixture is heated with the cream/milk mixture - until "it thickens enough to coat the back of a spoon".

I used two recipes - one with just plain cream and one with cream/milk.  Both times - the final mixture, when heated, would not thicken enough to coat the back of a spoon.  The mixture was heated once to 74° C and then again to 98° C.

I used chicken egg yolks the first time - they seemed too small.  I used duck egg yolks the second time.  Still no luck thickening.

I guess I would like to know when I should stop and take the mixture off the stove.  If thickening doesn't happen after 20 minutes on medium high heat with a temperature of 98° C - then what?
No, it doesn't have to thicken, at least not in the way custards like creme caramel thicken. "Coat the back of the spoon" is just a misnomer, a term known to experienced cooks which confuses everybody who learns to cook on their own. 

For "Coat the back of the spoon", read the comment riotburn left at the question What is the correct consistency of a cream soup?. It says: "The best way to describe 'coat the back of a spoon' is where if you dipped the spoon into the soup, take it out and holder over the pot, you'll find thin film (&lt;1mm thick)."

For the ice cream, heat to between 80 and 85°C. I've explained it somewhere else, but in short, this gives you enough thickening without overcooking. If it is really gelato and not French style, it should be thickened with starch anyway. Also, watch videos on the Internet for how to make ice cream, they show the correct consistency. I think Allrecipes had a good one, but their search is so strange, I can't find it and don't know if they have it or not.