If the document is straightforward, already prepared and in the correct form the Notary Public Lawyer is likely to need see you for a minimum of 30 - 45 minutes and complete it in one hour.
Obviously, it takes longer if the document is not straightforward, or if she/ he has to draw up the document specifically for your requirements or make a proper copy.
Many countries want documents to be countersigned by their London Embassies ( Legalisation ) and this will take several days unless you want to take them to the Embassy in person and wait in line. The fees and procedures for this vary widely but the Notary Public Lawyer should be able to give you a reasonably accurate idea of the time and cost.
Please telephone + 44 (0) 20 3178 5780 for an appointment.