This Lifehacker article:
https://offspring.lifehacker.com/why-you-should-encourage-your-kids-very-specific-obsess-1821246469
suggests special interests last 6 months to 3 years?? Hmm.

At the Engine Shed – South London, we know teenagers and adults who still enjoy building and playing with Brio and Tomy train sets; I know teenage autistic girls who role-play with Sylvanian Families, and grown-up men and women who are obsessed with Doctor Who.

These are not just hobbies. Special interests can be used to help learning and employment. If schools allow special interests to be the subject of school projects, they get amazing results from the children through their detailed knowledge and engagement. Autistic children can also develop other skills through their special interest – I have met a number of autistic teenagers who are developing amazing photographic and video-making skills by recording their interest in trains. These are all valuable “transferable skills”, to use the employment jargon. Special interests should be encouraged and supported.

Special interests last *how* long?