Incarnate threads and SSA's
Once the first time you ever complete the episode then once every WEEK across all episodes after that I'm afraid.
See the Wiki for more details. I'm not sure I expressed it clearly and the Wiki says it better than I can.

In Paragon City since June 2005.
There is also a weekly reward of 1 Astral Merit which you can convert on that character to 4 threads or send off to your account to be claimed as 4 threads via Astral Christie in Ouroboros.
But as Feefa J wrote each SSA gives you one reward one time as the first time reward for it but then all of them lump together in the once per week per reward timer.
So if you have not run any of the SSAs with your 50, run them all and get the first time reward for each of 10 threads and then run one of them again and choose the weekly reward of 10 threads.
total kick to the gut

This is like having Ra's Al Ghul show up at your birthday party.
Excellent!
Thanks for the help and the good advice.
@Roderick
Hello,

I've been running the new DA stuff and love it. But I need more threads.
I've heard that running SSAs will allow me to select threads as a reward but that I can only do so once a week.
So, does that mean that if I run 3 SSAs on Sunday I can get 30 threads? Or do I have to run an arc on Sunday, then another on Monday, etc in order to get 10 threads each time?