I think you have to consider where the player was picked as to whether the player was a bust, and if a bust, how bad a miss was it??
1. Any pick that doesn't make the rookie year final 53 man roster was a bust, at least initially. The caveat is that if you get that player signed to the practice squad then it is essentially the same as making it to the 53, so they still have a chance.
2. If a player drafted in rounds 4 through 7 plays at at a standard position that player must at least serve as quality primary serviceable depth, with at least occasional flashes that they could eventually be a starter. Failure to meet this would be considered a bust. If the player plays a specialty position such as kicker, punter or long snapper then that player must be a starter in his rookie year regardless of round. If a player drafted in rounds 4-7 is a special teams phenom, then they would not be considered a bust.
3. 3rd round player. Should be fighting for a starting position within the first year, serve as a strong backup, and eventually steal the starting position away from the players in front of him at his position by the end of his 2nd year.
4. 2nd round player. This is the high value area of the draft. Great potential at a much cheaper cap cost than a 1st rounder,. This player should start as a rookie, or at the very least be in a dog fight for starting position, a strong reserve and rotational player as a rookie. If you draft a guy like Jaylon Smith, then he should be a first round talent and be judged by first round criterion set forth below after his red shirt medical year. It is not worth it if Smith only performs at a round 2 level since we have already pissed off one year of his contract.
5. 1st round player. Should play well enough to start his rookie year on about 70 percent of the teams in the draft. Not starting in the player's rookie year is unacceptable unless it is a position which requires extra training (QB) or there is such a strong player in front of him that the pick was a luxury pick for the team acquiring that player. In the event that player doesn't start but should start because of talent level, but is buried on the depth chart due to exceptional players in front of him that player would not be a bust so long as the player eventually bumped the ageing player off of the roster and secured a starting position.
I think these are the minimum criterion for a player to not be considered a bust.