It all depends on the content and encoding (click here for more information about encoding)!
If you send in GSM mode each SMS can contain up to 160 characters.
In Unicode mode (with special characters like "€" or "Ê"), each SMS contains a maximum of 70 characters.
You can send a long text message that contains up to 8 SMS messages : it's called "Long SMS". Messages appear as a single SMS message on your recipients' phones.
In other words, if your SMS is too long it will still be sent (but will actually cost you several credits).
Tags: encodage, encoding, gsm, length, longueur, sms, unicode
Note: if you send long SMS messages, the number of characters per message is reduced from 160 to 153 in GSM mode and from 70 to 67 in unicode mode.