There is no common valid formula, but there are some recommendations from sport coaches how many training hours per year you should invest per sport:

(these values are from the brilliant book of Joe Friel)

Triathlon

• Ironman 400 – 1.200 hours / year
• Half Ironman 300 – 600 hours / year
• Olympic  Triathlon 200 – 600 hours / year
• Sprint Triathlon 150 – 400 hours / year

Duathlon

• Middle distance 300 – 600 hours / year
• Short distance 200 – 600 hours / year

Running

• Marathon 400 – 1.200 hours / year
• Half-Marathon 300 – 600 hours / year
• 10 km (6 mi) runs 200 – 600 hours / year
• 5 km (3 mi) runs 150 – 400 hours / year

Biking

• 150 – 300 km (90-180 mi) bike races 400 – 1.200 hours / year
• 90 – 150 km (50-90 mi) bike races  300 – 600 hours / year
• 25 – 90 km (15-50 mi) bike races 200 – 600 hours / year