「不満{ふまん}を吐{は}き出{だ}すだけ吐き出せば、スッキリするだろう。」
There are basically two patterns to this verb-だけ-verb construct.
1) 「Verb in Attributive Form + だけ + Same Verb in any form」
meaning "to [verb] to a degree". That degree is generally fairly limited.
2) 「Verb in Potential Form + だけ + Same Verb ...
@snailcar Ah, here it is. :D I see the meta post. I've added it to my list of sites interested in testing it. :D I'll try to set aside some time next week after I've caught up from the holidays to look at it.