The usual academic definition for "incel" is someone who fails to obtain sex despite trying for 6 months.
The 6 months may be considered arbitrary, but some time frame must be chosen, it is not realistic to accept someone as "incel" if he's been trying for a couple days; also it makes no sense to wait until he's 100 years old ("life begins at 80 bro"

).
That said, I think many responders to this thread here are confusing "
incel" with "
permavirgin". A guy may be an incel without being a permavirgin (r/FA30plus is full of guys who had sex once or twice in HS or college, and are now sexless for several years). He may also be a permavirgin without being an incel (e.g.: a volcel like Tesla or Newton, a monk, priest, sadhu, fakir, ascetic etc.)
A problem with the 6-month time is that if those 6-months fall with the winter in its middle, in a very northern place where people don't socialize outside much (e.g. Siberia, Canada, Finland etc.) the situation is very different from 6 months with summer in the middle, which is when there are open-air festivals, and people are more open to seeking partners for sex and relationships.
I would consider a guy who tries for 1 year without any success a fellow incel
IF he's graduated from college or older than 23. There is no way I would consider a 17 yo guy still in HS an "incel" because life can change a lot once in the following years (college experience, travel, alcohol, moving out, road trips, open-air festivals, etc. are verbotten for a 17 yo HS kid, but not for a young adult).
It's only after a person "settles" into the routine of adult life that the 1 year time frame should start to count.