The timers are independent. A way to slightly grow your collection as you fill them ...
Before turning in anything, check BOD Status and get a new BOD from the NPC
Now, as you turn one in, check BOD Status again and get another.
For any filled BOD run on that character you just got 'n'+1 BODs in return.
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One thing that really needs tweaking (IMO) is Tailoring. You really are not going to get rich doing those as compared to Smithing. I was shocked at the disparity between them.