How strong should temporary passphrase be?

[smesv]

I want to protect against brute force guessing my passphrase I added to 24 word seed in case it gets stolen. Is it hard to brute force with computer if phrase is too simple? How many guesses can be made per second?

[1499874318]

1499874318

[pooya87]

first to clarify, when using a wallet you set the password on your wallet file not the seed, the seed may be written in the wallet file and it will be encrypted with the password.
when writing down your password it can be stolen and used to gain access to the funds, the password you set on your wallet file doesn't change this.

for strong password you can read this https://en.wikipedia.org/wiki/Password_strength and find a lot more articles about it. but in summary, it should be
- long
- containing uppercase and lowercase letters, numbers, symbols
- it should not be regular words such as words of a poem, a popular sentence or something similar.

simple example using a random online tool: -4oC6vY317Z6S)s

[vh]

I added to 24 word seed in case it gets stolen. Is it hard to brute force with computer if phrase is too simple?

If you are talking about BIP39 each word is derived down to only ~11bits of security for a total entropy of 2^256. 
As long as you use on average 2 letters per word, I believe brute forcing it will take the same amount of time as a "difficult" to remember 24 word phrase.

[smesv]

I was talking about passphrase aka 25th seed word. How complex shoudl it be t oprotect if someone finds seed words and tries to brute force 25th word.

[EastBirth]

It really doesn't matter. The ones you are given are so random. How will anyone guess that many words and in the right order unless it's a sentence ? Don't worry about it.