My recommendation for the best way to guarantee freshness takes just a little bit of extra time and effort, but can significantly make a difference. As mentioned in this thread already, it is not just temperature, but oxygen exposure AND humidity that all combine to oxidize nutrients.
I will make a list from good to best and give my personal opinions. (Based on my products and my results from storing stuff in different ways and sending to the lab for analysis at different time increments)
One MAJOR comment I would like to make is that all Vitamins do not degrade equally, so you are not just loosing potency, you are screwing up your important vitamin ratios, such as those between the fat soluble vitamins.
This will vary from supplement to supplement based on the type of each vitamin used.... spray dried, micro encapsulated, chemical form..... there are a LOT of variables. Just getting to know how MY formulas react has been like getting to know a person over time.... I can't guess about anyone elses, so these comments below will be based on my ingredients.
For ME, (and probably most others) Vitamin E is the first to go, and vitamin A is the next to go.....
For example..... I often look at the extrusion stability reports of a particular vitamin form to estimate how it will hold up. Extrusion uses a LOT of heat in the process, so if you are making extruded foods, you have to formulate with levels that you would not feed at because of degredation.....
For example, my Vitamin A post extrusion is rated at about 65-70%... that means if I make a powder with 10,000 iu Vitamin A, after it is extruded, it will only be about 6,500 vitamin A..... And the Vitamin D3, post extrusion is about 95%..... so if we start with a 10:1 ration in our formula, we are going to end up with a 7:1 ratio......
Then over time, the Vitamin A breaks down 30% faster than the D3 and after a year, we could have a 1:1 vitamin A to D ratio

Then, besides heat, oxygen exposure and humidity can have different effects on how the ratios end up over time because for example, at the same temperature, but in a wet environment.... the Vitamin D might be less stable than the A.....
The difference between forms of vitamin C, can mean total oxidation in a matter of weeks vs. up to a year...
So the important thing is to do whatever you can to keep stability across all of these variables.
1. Using a package in a herp room where the ambient temperature is 80 degrees like you would find in a room full of diurnal lizards such as bearded dragons or uromastyx,....
In these conditions, you have heat, oxygen, and some humidity all at work.... I would say that after 3 months on a shelf in a herp room..... especially when half the rooms I visit have the supplement sitting on top of a cage with a basking light and the surface temp of the cage it is on is often well over a hundred degrees.....
2. An amphibian room....... less heat, but more humidity...... 3-6 months....
3. Non herp room..... opened a couple times a week..... 6-12 months.
4. Refrigerated and used regularly (taken in and out of refrigerator to use.....
and used every day... 6-12 months.
5. Unopened at normal room temps....... 12-18 months......
6. Unopened and kept in fridge 18-24 months.
Don't forget, that you never really know how old it is before YOU GET IT!
So how do we use the stuff and keep it the freshest?
Easy, Keep your bulk container in the fridge and just quickly take out what is going to last you about a month. This will avoid the condensation you get when you go from the fridge to the herp room and back all the time.
Just use an old container to keep in your room or find a small suitable container...
Another note, One reason I went with the bags in the beginning is that when you close it you can get out nearly all the air..... while in a jar, the less product, the more air....Oxygen absorbers will make a difference, but they are really designed for long term sealed use as just opening a jar for ten minutes is enough to kill the absorber..... so you will need a LOT of them, or be busy recharging them in the microwave.....
I need to get out of the house.... I hope some of this makes sense......
Allen