Not long, but there's plenty of options afterwards - coaching, personal training, sports administration etc. Let alone all the random jobs that everyone else looks at. Anyway, that's a red herring. The market decides the pay rate and the employer decides if they want to enter that industry. People go into jobs which don't pay much for a variety of reasons - not the least of which is that they want to do it. Also plenty of people change careers midway through life. And you can study part time by distance to set yourself up with a career after sport while you are playing.
Having to retire from playing in you mid 30s doesn't mean you should get 45 years pay crammed into 15 years work. Unless your skills mean that that the market pays you that much (for instance, EPL stars)