I'm actually not sure. I've been able to do it since the first time I tried. I thought it was like 3k or something though... Don't quote me on that. I don't know if you need that many in your account or if you just need to have earned that many lifetime though. I suppose I could spend all mine and find out.
I've spent some on a few small things, plus I've used them to play in some of their big FPP satellites - I actually used 10k to play in one of their WPT PCA satellites (or was it 2? I think it was 2). I still have 7k+. I know that's nothing compared to the high rollers, but for as little as I play on Stars, it's a lot. I can easily clear one of their $120 bonii in just a matter of days, thanks to them giving points for tourney play. I remember furing the WCOOP last year, it worked out to them pretty much paying my first $120 in tourney entry fees.
Thanks for getting that info there, RR. Good to know!