How long is it estimated that it takes for glass to naturally degrade in nature?

Glass bottles 1 million years

The next issue is how long it takes for the material to actually break down. In nature, different materials biodegrade at different rates. A leaf takes approximately a year to become part of the forest floor. An iron shovel, on the other hand can take years to rust away to nothing, and a large tree can take decades to completely break down. Common sense tells us that any material will ultimately biodegrade, even if it takes centuries.

Here's how long it takes for some commonly used products to biodegrade, when they are scattered about as litter:

Cotton rags 1-5 months
Paper 2-5 months
Rope 3-14 months
Orange peels 6 months
Wool socks 1 to 5 years
Cigarette butts 1 to 12 years
Plastic coated paper milk cartons 5 years
Leather shoes 25 to 40 years
Nylon fabric 30 to 40 years
Tin cans 50 to 100 years
Aluminum cans 80 to 100 years
Plastic 6-pack holder rings 450 years
Glass bottles 1 million years
Plastic bottles Forever

https://www.activesustainability.com/environment/waste-planet-digest-how-long-it-takes-to-decay/

The one million years estimate is in ideal conditions.

Very difficult question to answer. It depends on the environment the glass is kept in. In the ocean, glass will degrade much quicker than it will on land!

Referenced from observation of smooth, worn glass on the beach.

Different environments degrade glass at different speeds.
http://education.seattlepi.com/long-glass-bottle-degrade-landfill-5235.html