Generally speaking, we recommend to landlords to start the advertising process around 4-6 weeks prior to the availability of the property. This will give you a comfortable amount of time to generate a healthy amount of interest, conduct viewings but also set up the tenancy without rushing the process.
Most tenants tend to start looking for their next month within the last 2 months of their previous tenancy, so by advertising within 4-6 weeks prior to availability you'll be able to attract tenants who are actively looking for their next move.
For larger properties, we tend to recommend to advertise closer to the 6 weeks mark as demand for such properties can require a bit more patience from the lettings market.