Approaching 150 hours, if I'm being honest, I would say that it took me until about 1oo hours to stop forgetting things. Getting distracted and completing checklists in my head seemed to be the root cause looking back on it, so now I say all the checklist items out loud and start again if I get distracted. Seems to be working OK, but I'm fairly certain that when I start down the path of a new endorsement, I'll probably have a few episodes again. Things I've stuffed up include:
1) Forgetting to advance the throttle while priming the engine with the fuel pump on the fuel injected lycoming. Flat battery resulted.
2) Forgetting to turn the fuel pump off after takeoff.
3) Forgetting to turn off the carb heat on short final...multiple times!
4) Perhaps the biggest lesson, just prior to pass overhead a class D airport for a left downwind, ATC advised to join right downwind right. I didn't expect it and it and it really threw me. By the time I made the turn I was almost over the field and proceeded to totally forget the rest of my downwind checks. Landed with the fuel pump off. If I had my time again, I would have just asked for the left downwind given how close I was to the field.
Anyway, fairly comfortable with what I'm flying now, but all of these experiences tought me to respect what I'm flying. Just because you can land it doesn't mean you can fly it safely. It takes a good number of hours to get comfortable with the ins and outs of each new plane I rekon.
As for forgetting to turn off the master, luckily the vacuum system makes enough noise to prevent that!