I am 35 weeks and this week have everything set aside in a box - my dh wants to physically pack the bag (so he knows where things are). Inevitably this means that there will be a delay in the actual packing of the bags until I am in labour, but at least I won't be rushing or relying on dh to guesstimate what needs to go in the bag at the time.
having said that - just have essentials in there for baby and me, not books etc or other 'nice to haves'.
I did mine at around 25 weeks,once viable. I'd say there's no such time as too early to have a small bag of essentials for you ready. I had bleeds in my first pregnancy (low lying placenta) and it is nice to have your own stuff. The pack it when in labour thing works for some people but DC1 was a category 1 section - so out in less than half an hour!
Needless to say DC2 came in a calm planned manner at 39 weeks but it made me feel better to be organised the second time.
I'm 36 weeks and just starting to get some bits together for mine (dc2). I'm having a planned cs but I suppose it wouldn't hurt to get it packed just in case I go into labour earlier. I think I'm being a bit lazy tbh!!!