How long will severe sunburn last? shins of both legs still extremely red post 1 wk. mottled, shiny.

Depends on severity: If legs got mildly or moderately sunburned, then usual home care for sunburns should help. People can try over-the-counter hydrocortisone cream for 4-8 days, while also moisturizing with their favorite non-medicated lotion or cream. Severe means different things to different people. Severe usually means the skin blistered off and new top skin will need to form, over 3-4 weeks (doctor can examine).