20 amp circuit breaker failed to trip
Infinite time.

Your 12 amp vacuum cleaner is probably labeled according to the maximum current it can draw (to be sure, look at its electrical specifications rather than the big marketing label some manufacturers put on the front to make the vacuum sound powerful).  It is unlikely that it was actually drawing 12 amps at that particular time.  If it was drawing say 8 amps, maybe the total load was closer to 22-23 amps.

If the total load was about 10-15% over the circuit breaker rating, it is expected that it would not trip.  Circuit breakers are rated for the current they will definitely allow for an unlimited period, and there is some tolerance on top of that depending on the ambient temperature (the hotter the breaker is, the lower the tolerance above labeled current will be).  We can assume the ambient temperature was less than 20C since you were using the space heater, and this gives roughly a 5% "bonus" to the current before the breaker trips.

If for some reason you want to make sure 22 amps cannot be drawn on that circuit, you should probably install a 15 amp circuit breaker instead.  But since this is recent work done by an electrician and the outlet is near the breaker, it's unlikely a cause for concern.  But do make sure the power strip plugged into the outlet is rated for 20 amps as well (or that you are using two separate strips with their own 15 amp breakers).