Who is dealing with the estate? Are there substantial assets? Is there a property to be dealt with?
If assets are insubstantial, you may not need a Grant of Probate. If you need a Grant of Probate, see below.
Have you appointed a solicitor? if yes, all the assets have been identitified and you are ready to meet the solicitor, swear the Inland Revenue Affidavit etc.,etc, you are looking at an approximate timeline of 16-20 weeks in the main Probate Office.
If you are proposing to deal with Probate yourself, I don't know the timelines but they will be longer than above.
mf