How many time can you use a free 14 day ps+ trial after creating an account?

I have already used one trial on my main account and used another trial on a second account. So how many time can I do this?

It may have changed in the meantime, but the last time I checked (January 2014) it is impossible to redeem the free 14 day PS Plus trial for an account that has had PS Plus before.

I've received 30 days of PS Plus for free in 2011 as an apology for the 2011 PSN hack, and haven't been able to redeem the 14 day trial since, nor did I ever redeem it before.

Whether this also applies to trial codes shipped in some games, I can't say (probably not). I've yet to see a PS3/PS4 game that ships with such a code.