I was issued an O visa multiple entry in Cardif 8th July 2009, i have done my 90 day visa runs to Laos and back and i am now on my last 90 day entry stamp from my last visa run which expires on 29th September 2010 , how many days in advance of 29th September do i need to go to immigration for an extension visa (Based on marriage to a Thai), The British Embassy are late in sending out my letter to confirm my monthly income , it should arrive on 1st September then i have all the required documents for the extension visa

Thanks in advance for any assistance

You can go up to 30 days before your permission to stay expires. Do it early, as you won't lose any days since it will start when your current permission ends. It gives you time to get additional papers etc if immigration should ask for it.

As Mario said apply within the last 30 days.