In what kind of recipes can I substitute stock for water?
Any recipe where the water is part of a sauce or is expected to be absorbed (including rice, couscous, and yes, pasta too), you can use stock instead to increase the deliciousness.  If you have an opportunity to add flavour, why waste it?

Of course, there are some caveats to consider when making the substitution:


Stock is going to contain a certain amount of gelatin (how much depends on how the stock is made).  This is often a good thing, leading to a richer mouth-feel, but if the thickening effect would be detrimental to your recipe, then don't use stock.
Stock can clash with other flavours, although less often than you'd expect, since it's effectively a mega-dose of umami.  In particular I'd probably avoid using it in recipes that are either very sour (sour meat is the taste of rancidity and highly displeasing) or very sweet (since the sugar will overwhelm the flavour of the stock).  So don't use it in your candies or in your pickling solutions, but any dish based primarily on meats, vegetables, and grains, is definitely fair game.
Because it's a meat product, it has a limited life span.  I wouldn't recommend substituting it for water in anything that's going to sit in the fridge for several days and/or be re-frozen, due to food safety concerns.
Oh, and I probably wouldn't use it in baking.  The effects of using homemade stock when the primarily role of the liquid is to develop gluten would be unpredictable, to say the least.  And somehow the thought of turkey-flavoured cake just doesn't sound very appealing to me.


As for the right ratio - that depends entirely on what went into the stock, how long it was simmered for and whether or not the stock itself was reduced prior to storing/freezing.  You're definitely going to want the same total amount of liquid, but if the stock is highly gelatinous then you might want to dilute it, and conversely, if the stock is very weak then you might want to reduce it.  There really is no golden ratio, but I'd probably stick with 1:1 for a "typical" stock.