My hardest decision in the past has often been when to decide a startup has failed and its time to move on. Having seen many startups both succeed and fail you'

My hardest decision in the past has often been when to decide a startup has failed and its time to move on. Having seen many startups both succeed and fail you're in a unique position to perhaps give some strong opinions on this. Do you have any sort of timelines/guidelines you like to look at as a gauge of success and when it's time to double down or dump it off the side and try again?