Once you decide on your aim - your real, definite aim in life - don't put an expiration date on how long you'll persist in going after that success.
If it's your true goal, you must decide tong after it until.
Until you physically cannot any longer.
Until the possibility of obtaining said goal no longer exists.
Until you decide to go about it in a different way.
If there's a date or amount of effort number of you-should-quit advisories that is your benchmark for ceasing your pursuit, it's probably not your true #1 goal.