How long is it safe to be on antibiotics (minocycline)? It's been around three years on and off but whenever I stop my acne comes back. Risks?

It's safe. I was on various tetracyclines for decades until my acne finally remitted around age 50. You also need to be on a topical agent such as benzyl peroxide and/or retinoic acid. Since you're a female, you may try spironolactone or the oral contraceptive pill. I'm glad you've gotten a good result. There's no guessing when your acne will go on its own; keep managing it until it does. Best wishes.