how much can refurbishing do for a Birkin?

when i took my bag in to nyc, they only needed to recondition it, not refurbish it, so they only charged me $40 and it took 2 or 3 weeks. the tip they gave me is never send in your bag for reconditioning/refurbishing when the seasons are changing. for example, march/april is very busy because that's when everyone brings in their winter bag to get cleaned up and they start using their spring/summer bags. or they bring in their spring bags to get fixed up in time for may, etc. december/january, mid-summer, etc. are all times with a little less traffic so your wait time won't be as long.