I have a AF-S Nikkor 50mm 1:1.8 G lens which I use for my Nikon D3100 and D3300 cameras. Today, I noticed that the focus ring can be twisted past the infinity and 0.45m focus stops. It can be turned indefinitely without ever hitting a hard stop. The focus scale indicator stops moving and the focus elements in the lens appear to stop moving, but the focus ring can still be turned without ever stopping. Is this typical, or is my lens defective? I've only had it for about a month.
Lenses such as the AF-S Nikkor 50mm 1:1.8 G use a type of motor to move the focus elements known as a Silent Wave Motor (SWM). The technology was first developed by Canon, who refers to it as an UltraSonic Motor (USM). It has since been adopted by many lensmakers and is known by such monikers as Supersonic Wave Drive (SWD) - Olympus, Supersonic Drive Motor (SDM) - Pentax, Supersonic Motor (SSM) - Pentax, Hyper-Sonic Motor (HSM) - Sigma, and Ultrasonic Silent Drive (USD) - Tamron.
The design uses rings vibrating at very high frequencies to produce the torque needed to move the lens' focus elements. Because these rings are not directly geared to each other, when the focus ring on an SWM lens is turned it allows them to slip in relation to each other without risk of damage to the focus motor. It is quite normal with this type of lens to be able to continue to move the focus ring indefinitely even after the focus elements in the lens have reached the end of their travel.
I noticed that the focus ring can be twisted past the infinity and 0.45m focus stops. Is this typical, or is my lens defective?
That sounds normal. The product page for your lens lists 0.45m as the minimum focus distance, and it's not uncommon for a lens to be able to focus a little bit beyond infinity. If the lens were designed to stop exactly at infinity (ignoring the cognitive dissonance in that statement), then any small imprecision in the mechanism would prevent focusing at infinity, autofocus systems would have a hard time focusing at infinity, and you'd forever be running up against a hard stop when shooting far-off subjects.