After how long, is it safe to take a blood pressure on someone who has had a mastectomy?

Other arm: Generally most patients who have had a mastectomy will have dependent lymphedema of the affected arm, due to lymph node dissection. You are perfectly able to take a blood pressure in the other arm.

Depends: Always try to use the good unaffected side first ---but if a few sentinel nodes removed it is okay if you have to but not by the automatic machine that goes to 300 first- do manually. If full lymph node dissection avoid that side at all cost use leg - calf any thing elsewhere as BP cuff used too high is worse then needle sticks done infrequently.