It really differs from firm to firm so nobody can give you a useful generalised answer to that. You're better off asking the cohort above in the particular firm that you will be training in at which point they received their confirmations last year.

Believe me, looking out for your (ie you, the trainee's) well-being or career trajectory isn't the topmost agenda on a law firm's list of priorities.

You should start applying elsewhere the moment you think you wouldn't be retained or have made the decision not to stay.

Part B is over and the next batch of trainees are probably beginning to fret over retention prospects; and so the cycle continues.

If it's any comfort to you at all, if at the end of the day you aren't retained, it is in all likelihood more a result of financial constraints than a lack of ability on your part. Think about it this way, an office photocopier machine has more value to a firm than a legal trainee with 0 years of working experience under his/her belt.