Boveda provides the cleanest and most purified humidity of all other products available on the market today. 8 gram Boveda are commonly used in smaller jars and stash tins. They can last 6+ months depending on how often the container is opened, you'll know it's time to replace when the entire Boveda begins turning hard.
When do you use a 8 gram Boveda? Whenever it fits in your storage container. Boveda will never humidify beyond the RH on the pack, so there's no such thing as "too much". You want to think in terms of using at least the minimum because the more you use, the more efficient Boveda works and the longer they last
Do not mix Boveda RH levels within the same container, and do not use Boveda in the same container with other humidification products. They will fight against each other and reduce the efficiency of the Boveda.