When is it safe for reconstructive surgery after mastectomy?

Depends. Reconstruction can be done at the time of the mastectomy (and give the better cosmetic appearance) or anytime after the scar has healed. If done later it is typically delayed until any chemo or radiation has been completed and your body has recovered from them. This may take 1-2 years depending on type of chemo and your reactionto it.
Depends. Reconstruction can be done immediate at the time of mastectomy or delayed done after all treatment is completed. Safety depends on your general health (ability to undergo general anesthesia) and the amount of scarring from surgery and radiation. I suggest you speak with your surgeon who can refer you to a plastic surgeon and explore the options.