It depends on the programme, for example a strength programme it would be when you 'own' the weight not the weight owning, meaning you can perform the specific exercise with perfect technique at your current weight when ever required. Then you work in the next weight. With a muscle building programme you are trying to stimulate muscle growth not strength so you may use a heavier weight earlier to rip muscle and so increase growth (with diet). The weight you use in a strength prog should be heavier than a gym style muscle pump prog due to less reps. Before I go too deep, I'd reiterate I'm not a strength and conditioning coach, just read a lot on the subject and have my own experiences of different progs over 20 years, of which the results are of my doing whether good or bad. Check out Strongfirst for proper advice from very knowledgable coaches.