The formal lease extension process can take around six months, although this can vary greatly. If you and your landlord are able to agree on the premium to be paid early on, it could take a little less time than this.
If you need to negotiate or an agreement cannot be reached, then the length of time taken can be much longer. By using the statutory lease extension route, the term remaining on your lease will stop running down when the initial notice is served on the landlord. This means that while the process is dealt with, the time left on the lease will not reduce. This can be particularly helpful if the remaining term is approaching 80 years. Below 80, the premium required can increase more quickly.