Replacement Driver Burn-in Time???

It really depends on the driver and how it is used, I would anticipate at least 200 hours before it really sounds "normal". With some drivers its not uncommon for it to take 400+ hours for break in, but typically the first 200 is the real rollercoaster- ups and downs and huge changes in sonic signature.

I'm picky and impatient. I always replace in pairs. That way the break in time isn't as vital and lot changes from year to year won't affect things. If it's "bad", consider getting another woofer and sell your single.