Is the continuity test limit resistance of a multimeter standard?
It is not a standard value from meter to meter. Different models will beep at different resistances. 

An example in the comments were the fluke models 175,177 and 179. In the datasheet for these, you can see on page 12 that the meter beeps at &lt;25 ohms.

The multimeter I use is a Tenma 72-7732A, and in the datasheet for this model, on page 35 it is stated that the beeper will sound for conditions less than 50 ohms. 

In the popular EEVBlog multimeter, page 25 of the Datasheet states the continuity threshold is between 30 and 480 ohms.

These few examples are enough to determine that there is no standard value. If the information cannot be found in the datasheet of your particular model, then getting a few resistors between 10 - 250 ohms and measuring them should be enough to tell you the threshold.

An easier way, as pointed out by @HarrySvensson in the comments is to turn a potentiometer/rheostat until the beeping stops/starts and measure the resistance.