The maximum length of a text message is 160 characters as long as you use standard 7 bit characters. Once you use a character that is not in the 7 bit encoding list, your maximum number of characters in a text message drops to 70.
When you write your message in the CheckMarket Survey tool you will see the number of remaining characters diminish as you type.
To help you out a bit, this article lists the characters that count as either 1 or 2 characters, as well as the ones that change your text message into a Unicode message.
7 bit encoding
The following characters all count as 1 character:
Some characters are counted as 2 characters, though. These are the following:
As long as you use any of the characters listed above your text will be a 7 bit text and the maximum number of characters is 160.
Unicode (UCS-2) encoding supports a greater range of characters and languages compared to very limited alphabet of 7 bit encoding.
If your message contains any characters not listed in the 7-bit alphabet above, UCS-2 encoding is automatically used. In this case, each message takes up a lot more space, thus the message length is reduced to 70 characters.