(How long) would Earth's atmosphere last without a global magnetic field?
Atmospheric escape is a topic with a long research history. It is complex and is being addressed with both measurements and simulations. 

For example, the question of atmospheric escape is still actively researched at Mars, and the MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft mission is for example dedicated to this topic. Mars is a planet without a significant global magnetic field (although it does possess crustal magnetic anomalies); in fact its planetary dynamo was active and then stopped about 3.6 billion years ago, which considerably reduced the strength of its magnetic field.
So understanding the evolution of its atmosphere helps to relate to what would happen if Earth would loose its magnetic field. The question of atmospheric escape at Mars is relevant concerning the fate of water (geological evidences indicate the presence of liquid water in the past, which suggests a warm and dense atmosphere able to support it at that time).

Besides thermal atmospheric escape, there is also non thermal escape, including removal by asteroid impact (in the past) and by interaction between the atmosphere and the solar wind leading to pickup of atmospheric ions (in the past and continuing today).

Some examples of references to the scientific literature about atmospheric escape at terrestrial planets (abstracts):
Outgassing History and Escape of the Martian Atmosphere and Water Inventory
or
Nonthermal escape of the atmospheres of Venus, Earth, and Mars