Hiya,
I too have a poorly 9wk old... slightly different situation to you in that we were admitted to hospital on tuesday as she needed oxygen to help breathing & was fed via a nasal-gastric tube for 3 days to help her conserve energy / stop vomiting back up every feed (she has a nasty bought of bronchiolitis).
As soon as I got to hospital, I started expressing, religiously every 3 hours day & night on both sides (she normally only feeds from one side at any one feed). I know it was probably overkill, but I figured I'd rather err on the side of caution & make sure my breasts were being stimulated regularly. I expressed both boobs until I couldn't get any more out using the pump.
She started breastfeeding again yesterday evening, so had gone almost 3 days without any breastfeeds at all. I was terrified that BF might have all gone tits-up (excuse the pun!) & we wouldn't be able to continue after 3 days with no BF. HOwever, I am pleased to say that I have plenty of milk for her & we are now discharged & home :)
Anyway, my advice would be that if she is still feeding fairly regularly but not taking what you would consider a 'full' feed at the breast, then once she's off the boob, express the rest til 'empty' (ie you cant get any more out using a hand pump - i know boobs can't technically be 'empty!). You could also express the other side too if she won't / doesnt take that. Essentially, just make sure your boobs are being stimulated as regularly as they were before she fell poorly.
I figure we will probably have some catch up to do in the next few days as DD fully recovers & starts feeding loads again.... but hey, its nearly christmas day so I can stuff myself silly & not worry too much about the excess calories!!
Good luck, its so stressful when you have a poorly baby x