You see - lots of different answers - no-one really knows.
But there seem to be two different questions here:
1: How long before any data you have built up is deleted after the trial period expires?
2: How long after the trial period expires before your username is deleted?
In my case, I was able to re-activate my username about 5 or 6 months after the trial period. I didn't really have any data associated with the account so I can't remember if that was deleted. However, two dormant .mac email only accounts which I subsequently resurrected came back with all the data intact and they had languished much loger than 6 months.
I have no idea if there's a strict deadline that Apple enforce about all this - why not ask them?