In this lesson
Hours are the wrong unit
Teams ask for a number of hours because hours are what they have. But a thousand hours of one camera pointed at one conveyor in one lighting condition is roughly one condition sampled a thousand times. It teaches the model almost nothing about tomorrow.
The unit that matters is distinct conditions. Different sites, seasons, times of day, weather, equipment generations, camera mounts. Ten hours spanning twenty conditions beats a thousand hours spanning two. And within any hour of video, adjacent frames are near-duplicates - thirty frames a second is one scene sampled thirty times - so labeling a frame every few seconds captures nearly everything the footage has to teach at a fraction of the cost.
Count instances, not frames
For most detection work the practical floor is a few hundred labeled instances per class, and a few thousand before you would call it robust. Note that is instances, not images: one frame with fourteen insulators in it is fourteen instances.
This is why footage of a dense scene is worth more than footage of an empty one, and why an hour of gate camera at shift change is worth more than an overnight hour of the same gate.
Rare classes set the schedule. Your timeline is governed by the least common class you promised to detect, not the average.
Sample the edges deliberately
Left alone, any archive is dominated by the ordinary. If you sample randomly you get a dataset that is excellent at the common case and blind at exactly the moments you built the system for.
Pull deliberately for the awkward: dusk and dawn, rain and glare, the dirty lens, the repositioned camera, the new equipment model. Those frames are worth several times their number.
Budget for the second dataset
The first dataset gets you a working model. The second one, assembled from what the first model got wrong in the field, is usually what gets you an accurate one.
Plan for it from the start. Teams who treat labeling as a one-off purchase are the teams who find themselves rebuilding from scratch in month nine.