It never completely dissipates during typical cooking processes. If you're cooking for someone allergic to alcohol, best to leave it out altogether rather than try to simmer or flame it off. If you're talking about getting enough alcohol out to prevent drunkenness, obviously that depends on the how much you had to begin with. Rule of thumb: The longer you simmer the less alcohol you'll have. Presumably you already knew that.
Another rule of thumb: If you're cooking for someone in AA who says "no alcohol," that means no booze, as in none. That doesn't mean -- if you cook all the alcohol out of the booze I'll be alright. Don't mess with someone's program.
Let's not get the whole debate on The True Meaning of Boiling Points going again. But, the rate of evaporation depends partly on concentration. That is, more alcohol will vaporize out of a more concentrated solution more quickly, but as the solution becomes less concentrated it gives off alcohol at a lower rate. It's a kind of "half life" thing, if you know what that is. FWIW, ethyl alcohol (the kind you drink) boils at a skosh over 78C, which is itself, a skosh over 170F. But, the hotter the solution, the faster the rate of dissipation.
Returning to the practical level: It typically takes about 5 or 6 minutes at a simmer to get the "raw" taste out (longer with wine), and about a minute at a rapid boil (as in a deglaze for a pan reduction.