I’d say it is relative to the event you’re training for. Long runs provide aerobic support for your race specific training. I like to have my long runs be at least as long as the mileage i’ll reach during my race prep workouts... so when training for a 10k, workouts will be ~6.2 of race pace+ 1.5 warmup + 1.5 active recovery between sets + 1.5 cool down, so I’d want my long run to be 11+. 7-8 would be a decent starting point for 5k training.
From a physiological standpoint, I’ve seen training articles recommending 1.5-2 hours b/c of some unique metabolic stimuli (e.g. your fast twitch start getting recruited later in the run and it helps spur adaptations that make them more aerobic)... maybe someone more knowledgeable will chime in.