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.

Related Questions

When is it safe to take a blood pressure on someone who has had a mastectomy?

Yes. If possible doctors advise not use involved side aren , to take b.P or to draw blood.If needed can be done . Read more...
Depends. It is important for you to find out how many lymph nodes were removed. The opposite side can be used immediately. The older teaching is not to do blood pressures on the side where you had operation. The long term studies have not shown this to be a problem. I recommend speaking with your doctor for specifics. Read more...