The cuts are rumoured to be 25% over 4 years, so that's about 6% a year. From all the waste in the public sector, those cuts could be relatively painless. (Just look at the savings Philip Green identified).
There will often be no savings (or even a slight extra spend) in the first year. This is because of redundancy payments, costs of existing contracts, etc.
When will they start? Depends on the complexity of each cut. If its easy to cut, bet on the cuts coming quickly. If its more difficult (or politically damaging) the cuts will come slowly (or not at all).
Below is the initial timetable as taken from the MOD Army website. I believe it will take some time, however, for all the consequences to emerge in detail:
On Monday 18 October, the Government will present the new National Security Strategy that underpins the decisions being made on resources. This will set out an analysis of the threats and risks that we face, and a vision of Britain's role in the world going forward. This is essentially the first part of the SDSR.
On Tuesday 19 October, on the basis of that strategic backdrop, the Government will present the second part, or main body of the SDSR, notably the decisions being taking on resources, in order to deliver a safe and secure Britain and ensure we can fulfil our international responsibilities.