By up to how long before submitting an annual extension of stay application to Immigration do you need to obtain a proof of income letter from your Embassy (as distinct from a balance confirmation letter from your bank)? My gut feeling suggests 4 weeks max, but is this the right order of magnitude?

That should do it. It is not as strict as the bank balance letter. Some even do with the same letter each year, for several years. But immigration now wants to see a new letter every year.

Normally the immigration accept a proof of income letter from the year you are submitting your annual extension of stay.

Thanks, chaps.