We have worked with Clients who have achieved ISO 27001 in 3 months and others who have taken a year to get it. A lot depends on what you already have in place and whether this already meets the standard required by ISO27001. If the business needs to improve its practices to meet the standard this can take a lot longer.
ISO27001 Advantage provides the guidance to achieve certification within six months. If you are able to tailor documents and get the evidence to support the certification in place, then you can achieve the certification quicker.