In a report by the International Business Times entitled 'SSDs lose data if left without power for just 7 days' they report on a presentation given by Seagate's Alvin Cox, also chairman of the Joint Electron Device Engineering Council (JEDEC), they write;

"The standards body for the microelectronics industry has found that Solid State Drives (SSD) can start to lose their data and become corrupted if they are left without power for as little as a week."

They go on to report;

"Security firm KoreLogic is concerned that far too many people are now using SSDs in both consumer and enterprise applications, which is clearly not a great idea if the data is important and might be needed for a longer period that three months."

Many of the Expert teams use SSD for their main system drives, few of us use them for storage of critical data. It had been thought with the reduction in the cost of SSD that it might be a good medium to store project archives, that idea may now need a rethink.

Our advice is to make sure that any critical data is backed up to mechanical hard drives.

You can read the full article here

However an alternate view is cited by IT World who have run a response to the original article entitled "No, you won't lose data on your SSD if you leave it off for a week." You can read their response to it here