I find that maintaining the humour for more than 5K words is manageable if you write in installments, for me, at least. And I guess it depends on the plot and characters.
(Also, it definitely helps if you have a writing partner. rampant_chaos8 and I have churned out several 10Ks worth of Peter/Sylar/ZQ/Milo crack over MSN, and it still makes me laugh.)
no subject
(Also, it definitely helps if you have a writing partner.