Radiation dermatitis or radiation-induced skin reaction (RISR) usually lasts about 2-4 weeks.
Your skin might well get worse for a while before it gets better, but will generally be healed over and less sensitive to irritation after a month.
Note that skin at the radiation site might look different after it’s healed for quite some time. Post-radiotherapy skin can be affected by hyperpigmentation where skin is a different colour, texture and tone to surrounding areas (which can mean darker or silvery patches on skin of colour, or tanned, red or pearly-pink areas on white skin) for a good while; up to six months is common, but some changes can last much longer.
Take care of delicate skin while healing by moisturising regularly with good, unscented, non-irritant products. If you're worried about pigmentation, scarring or changes of skin texture after radiotherapy, try Balmonds Rosehip Scar Oil, which is rich in regenerative rosehip, anti-inflammatory calendula and chamomile, and calming lavender, to even out skin tone and smooth uneven patches.
See How To Look After Your Skin Before, During And After Chemotherapy for a detailed guide to looking after your skin during radio- and chemotherapy.
Recommended products for looking after skin during radiotherapy:
Daily Moisturising Cream with calendula, shea and hemp seed oil, from £13.99 for 100ml
Intensive Hand Cream with seabuckthorn, calendula, chamomile and shea, from £10.99 for 50ml
Cooling Cream with menthol, aloe, lavender and calendula, £19 for 100ml
Skin Salvation with hemp seed, beeswax & calendula, from £7.99 for 30ml
Balmonds Rosehip Scar Oil with rosehip, lavender, calendula & chamomile, £18.99 for 50ml