The links provided above seem to suggest that although aspirin isn't recommended whilst breastfeeding, there's no evidence that it's actually harmful, especially if you've only taken one dose (2x200mg I assume?). The half life of aspirin in your blood
after absorption is 13-19 minutes, with salicylate (the first metabolite of aspirin) being 3.5-4.5 hrs (link
). You won't have any aspirin remaining in your circulation this evening (about eight hours later?), but you will still have the metabolites. Accumulation in other tissues and breast milk is another issue entirely. Though like I said there's no evidence that this is harmful.
Regardless of whether aspirin in breast milk reaches a peak after 3 hrs or 9 hrs, you can presumably expel the breast milk first this afternoon sometime to get rid of the aspirin before feeding your baby later? (have to admit I'm neither a woman or parent so I don't know about the ins and outs of this - how long it takes your body to erm, fill the milk jugs so to speak, etc.). You may want to do more research about this if you're worried about it, there's a load of info on the internet and these sites may be a better place to look than Toytown, but like I said it doesn't seem to be harmful.
Disclaimer: Not a medical professional and this isn't medical advice, what you do you do at your own risk, I don't accept any liability, etc