How long does it take too switch a mortgage?

my friend currently has a house and a bungalow, no mortgage on house and has mortgage on bungalow. he is changing too a buy to let mortgage by putting half mortgage on bungalow and half mortgage back on house (re-mortgage) he has sent all paperwork back and paid admin fees etc etc, how long does it take too hear back from the bank too say it has been switched over?