Bow:
If playing only 30-60 min daily: Rehair every two years
If playing 60 plus min daily: Rehair yearly
Instrument:
If playing only 30-60 min daily: New strings every two years
If playing 60 plus min daily: New strings yearly
Never throw out old strings unless they are broken. Label them with the date removed and put them back into the envelope they came in or another baggie/envelope. Keep them in your case or safe at home in case of emergency. Always have at least an extra A or E string in case of accidental breakage. Keep in your case for emergencies.