In informal speech and writing, you can use the indefinite “you” all the time, as this sentence illustrates. It is convenient; and, unlike the indefinite “they,” it is fairly clear to whom the “you” refers: It refers to you, the person to whom I am writing or speaking. Moreover, because “you” is the same in both the singular and the plural, it is doubly useful and applicable.
Be aware of two subtleties:
In formal writing and speaking, “you” should be avoided. Use “one” instead. This is the indefinite third person form, and it is a substitute for “a person” or “anyone.” For example, you might want to say, “You never know . . . .” Well, in formal style, it would be better to say, “One never knows.” This may be a holdover from the old days, when formality and politeness required speaking to others in the third person (see Levels of Speech), or it may simply be good manners. It is truly a question of style. Politicians, as an example, will use “you” all the time because they want to appear “closer to the people,” even though it may appear to us as insincere and manipulative. Academicians will use “one” ad nauseam because it seems more formal, erudite and remote.
As a general proposition, use “a person” or “one” instead of “you” when you are not directly referring to the person to whom you are writing or speaking. If, in a letter or email, you provide a recipe or any “how to” instructions, do not use “you,” if the directions are generic, that is, they apply to anyone and everyone. Use either the imperative, or “one,” or passive voice.
Most well-formed instructions are in command form: “Take the first right, then bear left . . . etc.” I knew a woman who used to give recipes in this format: “First you take your pot, and you put it on your stove . . . then you measure your water . . . etc.” That is not good form, even informally, since this “you” is absolutely as indefinite as the indefinite “they.”