I think the time frame you've been given is good advice. Do your exercises religiously and ask your surgeon for a recommendation to a good oncology physio should you need it if the axillary area tightens. I'm not suggesting there will be a problem but if you have the info you can get on top of it straight away if there is - also find out from him/her how soon after surgery you would be able to see a physio if necessary. These physios can be expensive but you can get a health care plan from your GP which brings down the price.