How long do these last?

It depends on the situation. In a sealed container with no extra source of moisture, indefinitely. In my incredibly damp garage about 30 minutes. They can be "reactivated" by baking in the oven and I have been experimenting with using low power settings in the microwave with some success. (full power causes them to "Snap, crackle, and pop). I also bought some of the little humidity indicator cards so I can monitor the humidity in my containers.

When you try to reactivate by placing them in oven or microwave do the little packets they are store in burn/melt?

We even tested our silica gel packets 300F for 4 hours.in the oven and there were no problems. However we recommend baking in the oven for 0.5-2 hours at 200-240F. You can also microwave for 7-12 minutes in DEFROST mode. * Start with the least amount of time and increase the time as needed. * Many microwaves do not let you choose time for Defrost mode. Instead, they want you to enter the weight and the microwaves automatically calculate the time for you based on the weight. You can enter several weights and choose the weight that gives you the closest time to 7 minutes. * Reactivation time varies depending on the types of packets, saturation level of beads, and types of microwave. (NOTE) This reactivation method does not apply to aiwa packets.

Are these Made in the USA?

What you see in the picture is what you get. There is no address for a manufacturer or statement of origin. If it was made in the USA, my guess would be that it would say so. The seller - Silicagel Factory seems to be a Chinese company and they also sell on Alibaba.

How do i know when to recharge?

5 gram pack of 50 paper packets that you inquire about only contain white beads. Therefore, you can't visually see the color change when the beads are saturated. You can use weight method for paper packets. If the packets weigh around 7-8 gram, it is time to recharge them. Thank you.