Adcomms are more interested in what you've done recently. You could just enter the estimated hours since HS graduation. Or you could use the Repeated feature and add more timeframes (to show dedication to a sport or music, eg), divided out by the HS years and even further by adding the childhood years before that. All the timeframes you choose to include will appear above the narrative box, each with their own estimated hours. If providing a fair estimation is difficult, you might instead insert a 999 or 9999 to indicate the number of hours involved is too high to matter. Or, as mentioned put in a 0 or 1, because it doesn't matter.
Also, Hobbies don't require a Contact to be provided.