longer review time for category 5

I'm adding and reviewing a large number of vocab each day, and would like to lengthen the time before I see category 5 cards again. (The maximum maximum gap of 7 days is way too short, as my new card learning gets swamped by reviewing old cards). I realise one option is to create multiple categories, but ideally I'd like to be working with the a large pool of cards and only seeing my category 5 cards every few weeks.

My ideal solution would be an item in the toolbar where you could assign a length of average length of time to each category (or at least categories 3-5) before seeing those cards again. But for now I'd be happy with a shortcut, if there's a single number I could change in the code or something.

For the record, I'm learning Korean vocab, and adding 200-300 a week. Thanks - and apologies if I've missed an obvious answer somewhere~