I would assume much longer. I have a recollection [i.e. I cannot build a google query to find a reference right now] of some poor schmuck working with a tribute band who asked him to send a text message of the lyrics to "Machine Gun" (a Jimi Hendrixs song). Something along the lines of 'please send us machine gun'. He promptly replied with the words and a few hours later was on the wrong end of an armed response team storming his house asking where the machine gun was.
At the time the only explanation he could think of was his messages were being intercepted but this was denied and the official explanation was he had supposedly sent the reply text to the wrong person who called the police. His phone logs showed he sent the reply text to the correct number but the police just dismissed this with no explanation.
Now we have a more plausable explanation. This has apparently been going on for a long time.
"Devine, who plays in a Clash tribute band in his spare time, had sent a message containing lyrics from The Clash's Tommy Gun to his lead singer who had forgotten the words to the song.
According to The Sun, the message read:
"How about this for Tommy Gun? OK - SO LET'S AGREE ABOUT THE PRICE AND MAKE IT ONE JET AIRLINER AND TEN PRISONERS"
The arrest has prompted speculation about how the message was intercepted. Police maintain that Devine's message went astray. They say he actually sent it to a woman in Bristol by mistake and it was she who alerted police to the content.
However, The Sun also quotes Chris Dobson, a terrorism expert, as saying that the interception clearly shows that GCHQ is monitoring all vocal and textual mobile phone traffic."
How many similar messages, that at first glance seem suspicious, are ant each day? If GCHQ was trolling for these automatically and forwarding them all to the local police, they would be overwhelmed. It seems much more likely that the reply went to the wrong destination, and a public spirited recipient panicked and notified the police.
At the time the only explanation he could think of was his messages were being intercepted but this was denied and the official explanation was he had supposedly sent the reply text to the wrong person who called the police. His phone logs showed he sent the reply text to the correct number but the police just dismissed this with no explanation.
Now we have a more plausable explanation. This has apparently been going on for a long time.
[Edit, for clarity, this was in the UK.]