I applied for naturalisation on 11th Aug, received the confirmation letter on the 14th and this past Saturday received the papers for Oath or Affirmation of Allegiance. We were under the impression that it would take 8-9 months for this to go through and didn't want to send our documents away for that long (you need both passports, both birth certificates, marriage certificate and divorce decree if you or your spouse are divorced), so we took all these papers to our solicitor who photocopied and certified them (cost was £20 plus VAT). Then I just sent them off to the Home Office in Liverpool, never, ever dreaming they would come back so quickly. Last night we went to the solicitor and he administered the Oath, signed & certified and I've sent that off this morning. I suppose there is always the chance that this will take longer to come back.
You can apply 3 years after getting your indefinate leave to remain.
Hope this helps,,,I'll let you know when I get the Citizenship Certificate back.