This answer to this question is highly variable. The types of free drinks for players can differ not only from property to property, but also from bar to bar within an individual property, and even from bartender to bartender at a particular venue.
Often, a level of personal discretion is involved in terms of what and how much an individual bartender or cocktail waitress will comp you, depending on how pleasant you are, how generously you tip, and heck, just the particular mood you happen to catch them in that day. We've certainly had varying experiences at the same bar and no doubt many readers have too.
That said, some defined policies are in place at many properties, which dictate the parameters within which this discretion is exercised. What follows is what we gleaned from the various cocktail waitresses, bartenders, casino hosts, food and beverage managers, and players with whom we spoke, which should give you some useful rules of thumb when navigating these waters (or beers, or vodkas).
Comped drinks are for customers who are actively playing. That means you must be gambling at the time you place your order. If you just lost $300 in the sports book and are now standing behind your wife who’s playing a slot machine, your wife may get a drink, but you may not. That's a pretty strict rule and it's up to each casino how strictly they want to enforce it. In some casinos, waitresses have been given warnings that go into their files — and even fired — for serving non-players. The casino sees this as theft, because they want to be paid for their drink, either directly or via gambling.
There generally isn't a minimum amount you need to play to be considered a player, so yes, you can put one nickel at a time in a machine and technically you're playing. However, cocktail waitresses quickly become adept at discerning the different between someone who's obviously pretending to play and someone who's really playing; the pretenders are often stiffs to boot.
Comped drinks are made with well or call liquors. Call liquors are more expensive than well, but cheaper than premium. For example, Absolut vodka can be served, but not Grey Goose.
If a customer wants a premium liquor, he can usually get it if he's playing in the pit or at the dollar (and up) denomination machines. If a customer is a high roller, playing in the high roller pit or slots, or playing $25/hand or more, he can generally get whatever he wants.
The really good stuff, like Cristal or Dom Perignon, is only available to high rollers and it's always a big deal, because a comp slip has to be filled out, usually by the pit boss or the player's host.
A lot of casinos don't serve frozen drinks in the casino, because it would take too long to blend them, especially considering the volume of drinks being served. If frozen drinks are served, they're usually made from those pre-mixed slushy-type machines.