Almost every answer to this question is given in characters, and characters are the wrong unit. A sixteen-character phrase can be trivial or excellent depending on how it was chosen, and no count of letters distinguishes the two.
The unit that works is words, from a known list, chosen by something that has no preferences.
The arithmetic, in one paragraph
Each word drawn at random from a list of N possibilities multiplies the number of guesses by N. Four words from a list of a few thousand already puts the total far beyond what anyone can try against a login form, which allows a handful of attempts before it locks. Six words is the usual recommendation for something that protects everything else, because there the attacker may have your encrypted file and unlimited time.
Beyond six, each extra word doubles-and-more the work for the attacker and adds a real cost for you, and by then two other things matter far more.
The two things that matter more than the count
Who chose the words. A phrase you composed yourself is drawn from the words you happened to think of, arranged the way language naturally arranges them. That is a much smaller space than the list you believe you used, and it is a space attackers model well. The arithmetic above only holds when the choice was made by something with no preferences: a generator, or physical dice.
Whether it is reused. A perfect phrase used in two places is one breach away from being a compromised phrase in both.
A word list is a catalogue like this one: finite, ordered, and public. Its size is not a secret and does not need to be. What has to be unpredictable is which cards were pulled, and that is the part a human hand does badly.
What decoration actually buys
Replacing e with 3, capitalising the first letter, appending an exclamation mark. Each adds a very small amount of unpredictability, because every cracking tool applies those exact transformations first. They are the most-modelled habits in the field.
Meanwhile they make the phrase harder to type on a phone keyboard and harder to recall under pressure, which is how phrases end up written on paper or quietly reused.
One more random word beats every substitution you can invent. It is also easier to remember, because it is a word rather than an exception.
Where this advice stops applying
Phone and laptop unlock. Those are protected by hardware that rate-limits attempts and enforces delays after failures. Unlimited guessing is not available to the attacker, so length buys much less than it costs. A long phrase typed fifty times a day is a phrase that gets shortened.
Anything with a working second factor. A hardware key or a passkey changes the shape of the problem more than any phrase length does.
Length is a cost as well as a benefit, and the cost should be spent where guesses are unlimited: the password manager master phrase, the disk encryption key, the backup archive. Those three, and then stop.
What to actually do
- Generate, do not compose. Your password manager can do it. Dice work if you would rather not trust the machine.
- Six words for the phrases that protect everything else. Four is fine for things behind a rate limit.
- Skip the decoration. Spend the effort on one more word instead.
- Never reuse the master phrase anywhere. It is the one that has no fallback.
And the honest limit of all of the above: a passphrase protects nothing if it is entered into a convincing fake page. Length has no opinion about where you typed it. That failure is far more common than a cracked phrase, and no amount of arithmetic on this page addresses it.
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How long should a passphrase be?
Count words, not characters. Strength comes from how many words were chosen and how large the list they were chosen from, not from how long the result looks. Four words drawn at random from a list of several thousand is already far beyond what an online attacker can try. Six words is the common recommendation for anything that protects everything else, such as a password manager master phrase or a disk encryption key. Adding a seventh word matters much less than the two conditions below.
Does adding numbers and symbols to a passphrase help?
Much less than people expect, and it costs you something real. Substituting a 3 for an e or appending an exclamation mark adds a tiny amount of unpredictability, because attackers apply exactly those rules first. What it reliably does is make the phrase harder to type on a phone and harder to remember, which is how people end up writing it down or reusing it. Another random word buys more strength than any amount of decoration.
Is a passphrase I invented as strong as a generated one?
No, and this is the part that decides everything else. A phrase you compose is drawn from the words you happen to think of, in the order they naturally go together, which is a far smaller and far more predictable space than the list you think you are drawing from. The arithmetic below only holds when the choice was made by something with no preferences. Use your password manager's generator or physical dice.
What is diceware?
A method for picking words with real randomness: roll physical dice, read the result off a numbered word list, repeat once per word. Its value is not the dice themselves but the fact that no human preference enters the choice. Any generator that draws uniformly from a known list gives the same guarantee. The dice just make it visible, and they work when you do not trust the machine in front of you.
Should my phone unlock code be a passphrase?
Usually not, and this is where the advice on this page stops applying. Phone unlock is protected by hardware that limits attempts and enforces delays, so a long phrase you type fifty times a day buys little and costs a lot. Reserve the long passphrase for the things that protect everything else: the password manager, the disk, the backup. Length is a cost as well as a benefit, and it should be spent where guesses are unlimited.